THE  BOLE  OF  DIFFUSION  AND  OSMOTIC 

PRESSURE  IN  PLANTS 

LIVINGSTON 


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L.  B.   Cat.   No.   1 137 


THE  DECENNIAL  PUBLICATIONS  OF 
THE  UNIVERSITY  OF  CHICAGO 


THE  DECENNIAL  PUBLICATIONS 


ISSUED  IN  COMMEMORATION   OP  THE  COMPLETION   OF  THE    FIRST  TEN 
YEARS  OF  THE   UNIVERSITY'S    EXISTENCE 


AUTHORIZED  BY  THE  BOARD  OF  TRUSTEES   ON  THE  RECOMMENDATION 

OF   THE    PRESIDENT   AND   SENATE 


EDITED  BY  A  COMMITTEE  APPOINTED   BY  THE   SENATE 

EDWAED  CAPPS 
STAKE  WILLAED  CUTTING  EOLLIN  D.  SALISBUBY 

JAMES  BOWLAND  ANGELL      WILLIAM  I.  THOMAS  SHAILEE  MATHEWS 

CAEL  DAELING  BUCK  FEEDEKIC  IVES  CAEPENTEE         OSKAE  BOLZA 

JULIUS  STIEGLITZ  JACQUES  LOEB 


THESE  VOLUMES  ARE  DEDICATED 

TO   THE   MEN   AND   WOMEN 

OF   OUR  TIME  AND  COUNTRY  WHO   BY  WISE  AND  GENEROUS  GIVING 

HAVE   ENCOURAGED  THE  SEARCH  AFTER  TRUTH 

IN  ALL   DEPARTMENTS   OF   KNOWLEDGE 


1243/ 


DIFFUSION  AND  OSMOTIC  PRESSURE 


THE  ROLE  OF   DIFFUSION  AND 
OSMOTIC  PRESSURE  IN  PLANTS 


BY 

BURTON  EDWARD  LIVINGSTON 

OF  THE  DEPARTMENT  OF  BOTANY 


THE  DECENNIAL  PUBLICATIONS 
SECOND  SERIES     VOLUME  VIII 


CHICAGO 
THE  UNIVERSITY  OF  CHICAGO  PRESS 

1903 


o 
VJ 


Copyright  1903 

BY  THE  UNIVERSITY  OF  CHICAGO 


ill  &a  State  College 


PREFACE 

With  the  ever-increasing  tendency  to  regard  an  organ- 
ism as  a  complex  of  physical  and  chemical  processes  which 
may  one  day  be  analyzed  and  understood,  there  has  neces- 
sarily gone  hand  in  hand  a  tendency  toward  more  and  more 
accurate  and  quantitative  investigation  of  the  physics  and 
chemistry  of  the  cell  itself.  Among  the  various  groups  of 
physical  and  chemical  phenomena  that  have  been  found  to 
play  important  roles  in  the  life-process,  and  which,  there- 
fore, have  been  interrogated  for  answers  to  physiological 
questions,  none  has  stood  out  within  the  past  few  years  as 
more  fundamentally  important  than  those  connected  with 
diffusion  and  osmotic  pressure.  This  field  has  thus  far  only 
been  touched  upon,  and  it  would  seem,  judging  from 
researches  which  have  recently  appeared,  that  the  best  and 
most  far-reaching  work  therein  is  probably  yet  to  come. 

The  present  volume  will  deal  with  the  past  and  present  of 
diffusion  and  osmotic  pressure  from  the  standpoint  of  plant 
physiology.  It  has  a  double  raison  d'etre.  First,  it  was  felt 
that  there  was  need  of  some  direct  and  not  too  exhaustive 
account  of  the  essential  physical  facts  and  theories  of  the 
subject.  The  interest  of  the  physical  chemist  here  has  lain 
mainly  in  the  light  which  these  phenomena  have  been  able 
to  throw  upon  the  ultimate  nature  of  matter  and  upon  elec- 
trolytic processes.  It  has  thus  been  difficult  for  the  student 
of  physiology  who  is  not  at  the  same  time  well  versed  in 
physical  chemistry  to  obtain  the  information  required  for 
the  prosecution  of  work  in  this  field.  Secondly,  it  seemed 
desirable  to  bring  together  in  a  general  review  the  literature 
of  this  subject  in  its  biological  aspects,  so  that  the  promising 
and  unpromising  points  for  future  research  might  become 


IX 


12437 


Peeface 


more  apparent.  The  volume  will  thus  naturally  fall  into 
two  Parts,  the  first  dealing  with  the  purely  physical  aspect 
of  these  phenomena,  and  the  second  attempting  to  present 
in  a  more  or  less  unified  whole  the  physiological  results 
which  have  so  far  appeared  in  this  connection,  together  with 
their  bearing  upon  each  other  and  upon  the  vital  problem  as 
a  whole.  Chapter  IV  of  Part  II  was  presented  to  the  Faculty 
of  the  Ogden  Graduate  School  of  Science  of  the  University 
of  Chicago  in  candidacy  for  the  doctor's  degree  in  1901. 

The  author  wishes  here  to  express  his  thanks  to  Professor 
C.  R.  Barnes,  of  this  laboratory,  and  to  Professor  Jacques 
Loeb,  of  the  Hull  Physiological  Laboratory,  for  much  valu- 
able aid.  Professor  Barnes  has  kindly  read  the  manuscript 
and  has  made  many  suggestions.  The  author  alone  is,  how- 
ever, responsible  for  whatever  new  departures  are  to  be 
found  in  the  book. 

B.  E.  L. 

The  Hull  Botanical  Laboratory, 

The  University  of  Chicago, 

October*!,  1902. 


TABLE  OF  CONTENTS 

Preface  ------ ix 

PART  I.    PHYSICAL  CONSIDERATIONS 
Introduction 1 

Chapter  I.     Matter  and  Its  States  - 3 

i.     Fundamental  Theories  of  the  Nature  of  Matter. 

a)  The  Atomic  Theory. 

b)  The  Kinetic  Theory. 

ii.     The  Three  States  of  Matter. 

Chapter  II.    Diffusion  and  Diffusion  Tension  9 

i.     Gases. 

a)  Simple  Gases. 

b)  Mixed  Gases, 
/n}    Liquids. 

(a)  Simple  Liquids.    *■ 
b)  Mixed  Liquids, 
in.     Solids. 

a)  Simple  Solids. 

b)  Diffusion  of  Two  Solids. 

Chapter  III.    Liquid  Solutions       - 16 

%  Liquids  in  Liquids. 

ii.  Gases  in  Liquids, 

in.  Solids  in  Liquids, 

iv.  Terminology  for  Solutions  of  Differing  Concentration. 

Chapter  IV.    Ionization  -         -        -        -        -        -  -      23 

i.     Of  Gases. 
ii.     Of  Solutes  in  Liquid  Solutions. 

Chapter  V.    Osmotic  Phenomena  - 25 

i.     Osmotic  Pressure  of  the  Solute. 

a)  Non-electrolytes. 

b)  Electrolytes. 

c)  Colloids. 

d)  Osmotic  Pressure  in  General. 

xi 


xii  Table  of  Contents 

ii.     Diffusion  Tension  of  the  Solvent. 
in.     Experimental  Demonstration  of  Osmotic  Pressure. 

Chapter  VI.      Measurement    and    Calculation   of    Osmotic 

Pressure    -        -        - 35 

1.     Measurement  of  Osmotic  Pressure. 

a)  Direct  Method. 

b)  Indirect  Methods. 

1.  Freezing-point. 

2.  Boiling-point. 

3.  Vapor  Tension. 

11.     Calculation  of  Osmotic  Pressure. 

a)  Non-electrolytes. 

b)  Electrolytes. 

PART  II.   PHYSIOLOGICAL  CONSIDERATIONS 
Introduction 47 

Chapter  I.    Turgidity -        -      49 

1.     Protoplasm  and  its  Limiting  Membranes. 
11.     Plasmolysis. 
in.     The  Permeability  of  the  Protoplasmic  Layers. 

a)  Test  by  the  Plasmolytic  Method. 

b)  Direct  Test  of  Penetrability. 

c)  Absorption  Test. 

d)  Test  by  Toxicity. 

e )  Test  by  Accumulation. 

/)  Test  by  Metabolic  Processes. 
g)  Outward  Permeability. 
h)  Variations  in  Permeability, 
iv.     Action  of  the  Protoplasmic  Membrane, 

a)  The  Filter  Theory. 

b)  The  Solution  Theory. 

c )  The  Chemical  Theory. 

v.     The  Nature  of  the  Osmotically  Active  Solutes, 
vi.     The  Maintenance  of  Turgidity  in  Spite  of  Permeability  to 

Certain  Solutes, 
vii.    The  Relation  of  Turgidity  to  Vital  Activity. 

a)  The  Retention  of  Form. 

b)  Mechanical  Support. 

c)  Growth. 

d)  Curvature. 

e)  Work. 

viii.     Summary  of  the  Chapter. 


Table  of  Contents 


Xlll 


Chapter  II.    Absorption  and  Transmission  of  Water     -        -      91 
i.     Absorption  of  Water. 
II.     Transmission  of  Water. 

a)  Water  Loss. 

(1)  Evaporation. 

(2)  Water  Pores  and  Nectaries. 

(3)  Exudation. 

(4)  Summary  of  Water  Loss. 

b)  Upward  Movement  of  Water  in  Trees  and  Other  Tall 
Plants. 

in.     Summary  of  the  Chapter. 

Chapter  III.    Absorption  and  Transmission  of  Solutes  -        -    115 
i.    Absorption  of  Gases. 

ii.    Absorption  of  Dissolved  Solids  and  Liquids. 
in.    Transmission  of  Solutes. 

^Chapter  IV.    Influence  of  the  Osmotic  Pressure  of  the  Sur- 
rounding Medium  upon  Organisms 124 

i.     Introductory. 
ii.    Presentation  of  Material. 

a)  Influence  upon  Growth  and  Form. 

b)  Influence  upon  Reproduction. 

c)  Influence  upon  Irritability. 

(1)  Changes  of  Irritability. 

(2)  Osmotaxis. 

d)  Analogy  between  the  Effects  of  High  Osmotic  Pres- 
sure of  the  Medium  and  Those  Produced  by  Other 
Water-Extracting  Processes. 

in.    Summary  of  the  Chapter. 

Index 145 


PART  I 
PHYSICAL  CONSIDERATIONS 


INTRODUCTION 

In  the  following  treatment  of  the  physical  phenomena  of 
diffusion  and  osmotic  pressure  no  attempt  is  made  to  be 
exhaustive.  Certain  aspects  only  of  the  present  conceptions 
of  these  matters  among  most  physicists  and  chemists1  are 
discussed,  and  every  discussion  is  presented  with  the  sole 
aim  of  clearing  the  way  for  the  physiological  discussions 
which  are  to  follow.  Thus,  for  example,  the  whole  subject 
of  atomic  and  molecular  weights  and  their  experimental 
determination  —  so  important  to  the  chemist,  but  not  pri- 
marily interesting  to  the  physiologist — is  entirely  omitted. 
Also  it  may  be  added  that  no  attention  is  given  to  a  his- 
torical treatment  of  this  part  of  the  subject,  the  excellent 
chemical  treatises  which  are  now  available  rendering  this 
unnecessary. 

In  the  present  Part  very  little  is  original  with  the  author, 
excepting  the  mode  of  presentation.  The  various  texts  and 
the  original  papers  have  been  drawn  upon  wherever  it  has 
seemed  expedient.     Footnotes  give  the  names  of  the  authors 

i  A  general  confusion  among  younger  students  with  regard  to  the  way  in  which 
these  conceptions  take  the  form  of  theories  makes  it  seem  expedient  to  call  atten- 
tion to  the  following  points :  A  scientific  theory  does  not  pretend  to  state  the  truth. 
It  may  sometimes  state  a  part  of  the  truth,  but  this  is  not  primarily  its  aim.  Its 
aim  is  to  connect  the  facts  together  in  the  most  logical  and  plausible  manner  pos- 
sible, and  thus  to  aid  the  further  advancement  of  our  knowledge.  Its  M  employment 
has  its  origin  in  the  organization  of  the  human  mind,  which  handles  abstract  truths 

much  less  easily  by  themselves  than  by  the  help  of  an  illustrative  image A 

hypothesis  [or  theory]  can  neither  be  proved  nor  disproved.     It  is  merely  a  tool 

which  is  rejected  when  found  to  be  no  longer  serviceable What  the  'real' 

nature  of  matter  is,  is  to  us  a  matter  of  complete  ignorance,  as  it  is  of  complete 
indifference."  (Ostwald-Walkee,  Outlines  of  General  Chemistry  [London,  1893J, 
pp.  5,  7.)  A  principle,  on  the  other  hand,  does  attempt  to  state  the  truth;  it  is  a 
generalization  and  induction  from  a  great  number  of  known  facts.  When  a  fact  is 
discovered  which  cannot  be  included  under  a  principle,  then  that  principle  falls  to 
the  ground  and  ceases  to  be  a  principle.  What  was  at  first  a  theory  may  at  length, 
by  the  accumulation  of  evidence,  come  to  be  a  principle. 

1 


Introduction 


to  whom  we  are  indebted  for  the  more  important  points. 
The  student  of  this  subject  will  find  the  following  standard 
texts  .very  helpful: 

Nernst,  W.  Theoretical  Chemistry  from  the  Standpoint  of  Avo- 
gadro's  Rule  and  Thermodynamics.  Translated  by  C.  S. 
Palmer.    London,  1895. 

Ostwald,  W.    Lehrbuch  der  allgemeinen   Chemie,    comprising : 
Vertvandschaftslehre,  Leipzig,   1887  ;    Stoechiometrie,  Leip- 
zig, 1891;  Chemische  Energie,  Leipzig,  1893. 
Solutions.    Translated  by  M.  M.  P.  Muir.     London,  1891. 
Outlines     of    General    Chemistry.     Translated    by    James 
Walker.    London,  1895. 

Manual  of  Physico- Chemical  Measurements.    Translated  by 
James  Walker.    London,  1894. 

The  Principles  of  Inorganic  Chemistry.    Translated  by  Alex- 
ander Findlay.    London,  1902. 

Remsen,  I.   Principles  of  Theoretical  Chemistry.    New  York,  1897. 

Walker,  J.    Introduction  to  Physical  Chemistry.     London,  1899. 

Reychler,  A.  Outlines  of  Physical  Chemistry.  Translated  by  J. 
McCrae.    London,  1899. 

Dastre,  M.  A.  "Osmose,"  in  Traits  de  physique  biologique, 
publie*  sous  la  direction  de  MM.  D'Arsonval,  Gariel,  Chauveau 
et  Marey.    Tome  I.    Paris,  1901. 

Cohen,  Ernst.  Vortrdge  fur  Aerzte  ilber  physikalische  Chemie. 
Leipzig,  1901. 

Jones,  H.  C.  The  Elements  of  Physical  Chemistry.  New  York, 
1902. 

Whetham,  W.  C.  D.    Solution  and  Electrolysis.   Cambridge,  1895. 

Kohlradsch,  F.,  and  Holborn,  L.  Das  Leitvermogen  der  Elektro- 
lyte.    Leipzig,  1898. 

Traube,  J.  Physico- Chemical  Methods.  Translated  by  W.  L. 
Hardin.     Philadelphia,  1898. 

Blitz,  Henry.  Practical  Methods  for  Determining  Molecular 
Weights.    Translated  by  Jones  and  King.     Easton,  Pa.,  1899. 

Hamburger,  H.  J.  Osmotischer  Druck  und  Ionenlehre.  Wies- 
baden, 1902. 


CHAPTER  I 

MATTER  AND  ITS  STATES 

I.       FUNDAMENTAL    THEORIES    OF    THE    NATURE    OF    MATTER 

a)    The  atomic  theory. — The  whole  structure  of  modern 
physical  science  is  based  upon  the  atomic  theory.    This  theory 
supposes  every  mass  of  matter  to  be  composed  of  numerous 
ultimate,  indivisible  particles,  called  atoms,  which  possess  a 
peculiar  attraction   for  one   another.     Atoms  differ  in  the 
amount  and  nature  of  this  attractive  force,  those  of  every 
chemical  element  being  in  this  way  different  from  those  of 
every  other;  but  all  atoms  of  the  same  element,  when  under 
the  same  conditions,  are  exactly  similar.     Owing   to  their 
chemical  attraction,  atoms  seldom  exist  free  as  such,  but  are 
prone  to  unite  into  groups,  thus  causing  the  neutralization 
of  their  mutual  attraction.     The  groups  so  formed  are  called 
molecules.     If  the  atoms  composing  the  molecules  of  any 
substance  are  alike  (i.  e.,  of  the  same  element),  the  element 
is  said  to  be  in  the  molecular  condition  —  as  opposed  to  the 
atomic   or   nascent  condition,  in  which   the   atoms  are   not 
united  to  one  another,  but  exist  free  as   such.     When  the 
atoms  forming  a  molecule  are  of  different  chemical  elements, 
a  compound  is  said  to  be  formed.     The  physical  and  chemi- 
cal properties  of  molecules  are  very  different  from  those  of 
their  component   atoms,  and   they   are   also  very   different 
from  those  of  any  molecules  which  can  be  formed  in  any 
other  way.     But  all  molecules  which  are  formed  of  the  same 
elements  and  in  the  same  manner  are  exactly  similar  under 
the  same  conditions.     It  thus  appears  that  the  smallest  par- 
ticle of  a  compound   which   can  exist  and  still  retain  the 
properties  of  that  compound   is  the  molecule ;  break  this  up, 

3 


Diffusion  and  Osmotic  Pressure 


and  free  atoms  or  new  groups  of  atoms,  with  new  properties, 
will  result,  the  original  compound  having  been  destroyed  by 
the  process  of  separation.  Atoms  may  also  unite  in  such  a 
way  that  their  mutual  attractive  forces  are  but  partially 
neutralized,  thus  forming  incomplete  parts  of  molecules, 
called  ions.  Under  certain  special  conditions  molecules 
may  split  into  two  or  more  ions,  and  some  of  these  cases  of 
ionization  or  dissociation,  as  the  process  of  splitting  is  called, 
have  proved  very  important  in  the  development  of  the  sub- 
ject of  osmotic  pressure.  In  some  cases  an  ion  may  consist 
of  a  single  atom  which  has  split  off  from  some  molecule. 
Briefly,  then,  acording  to  the  atomic  theory  as  now  made 
use  of,  the  nature  of  any  mass  is  dependent  upon  that  of  its 
component  particles,  these  particles  being  atoms,  molecules, 
or  ions.  The  same  mass  may  contain,  at  the  same  time,  all 
three  kinds  of  particles. 

b)  The  kinetic  theory  of  matter. — According  to  the 
kinetic  theory,  the  particles  composing  any  mass,  whatever 
their  nature  mav  be,  are  in  constant  motion.  This  necessi- 
tates  their  being  considered,  not  as  packed  closely  one 
against  another  to  make  up  the  mass,  but  as  separated 
from  one  another  by  continuously  varying  spaces.  The 
continuous  motion  of  the  particles  is  probably  for  the  most 
part  a  vibratory  motion.  They  are  supposed  to  move  in 
straight  lines  and  in  the  same  direction  until  a  collision 
occurs,  when  they  rebound  according  to  the  principle  of  the 
reflection  of  moving  bodies.  It  thus  becomes  necessary  to 
consider,  for  comparison,  the  average  distance  apart  of  these 
particles,  or  their  average  or  mean  free  path.  This  has  been 
demonstrated  to  be  much  greater  than  the  diameter  of  a 
single  particle. 

A  rough  conception  of  the  state  of  affairs  within  a  mass 
of  matter  may  be  obtained  by  comparing  the  mass  to  a  cage 
of  angry  bees.     The  insects  in  such  a  cage  fly  in  straight 


Matter  and  Its  States 


lines  to  and  fro,  striking  against  each  other  and  against  the 
walls   of  the  cage,  ever  varying  their  distances  apart  yet 
always  remaining  equally  distributed  throughout  the  cage, 
t.  e,  always  keeping  their  average  distance  apart  the  same.' 
Thus  far  nothing  has  been  said  of  any  restraining  force  to 
counteract  in  a  measure  the  motion  of  the  particles  and  keep 
them  from  flying  apart  indefinitely.      Such  a  force  might  be 
roughly  compared  to  the  walls  of  the  cage  just  referred  to, 
for  it  is  these  restraining  walls  which  prevent  the  indefinite' 
enlargement  of  the  swarm  of    angry  insects.     More  accu- 
rately, the  restraining  force  in  the  illustration  is  the  sum  of 
the  reactions  produced  by  the  several  impacts  of  the  movin- 
insects  against  the  rigid  walls.     There  is,  indeed,  such  an 
active  restraining  force  present  in  all  masses  of  matter;  it  is 
ordinarily  made  evident,  however,  only  in  liquids  and  solids. 
This  force  is  the  cohesion  of  the  particles  themselves.     It  is 
probably  akin    to  gravitation,  in    exhibited    larger  bodies, 
and  is  an  inverse  function  of  the  square  of  the  average  dis- 
tance apart  of  the  moving  particles.     That  is,  the  mutual 
attraction  exerted  by  two  particles  decreases  at  the  same  rate 
as  the  square  of  their  distance  apart  increases.     It  will  thus 
be  seen  that  this  force  becomes  negligible  at  a  comparatively 
small    distance    from   any    particle.     But    the    particles    of 
liquids  and  solids  are  so  near  to  one  another  that  their  cohe- 
sive force  is  sufficient  to  overcome,  to  a  certain  extent,  their 
energy  of  motion  and  to  hold  most  of  them  within  certain 
fixed  limits  of  space. 

The  science  of  thermo-dynamics  rests  upon  another  sup- 
position of  the  kinetic  theory  of  matter,  namely,  that  the  tem- 
perature of  any  body  is  directly  due  to  the  kinetic  energy  of  its 
vibrating  particles.  Since  the  mass  of  any  particle  remains 
constant,  and  the  kinetic  energy  of  any  moving  body  is,  at 
any  instant,  one-half  the  product  of  its  mass  and  the  square  of 
its  velocity  (KE  =  \MVl),  it  is  seen  that  the  average  kinetic 


6  Diffusion  and  Osmotic  Pkessuke 


energy  of  a  particle  varies  with  the  square  of  its  average 
velocity.  We  neglect  here,  as  comparatively  unimpor- 
tant, all  other  forms  of  motion  which  a  particle  may  possess, 
such  as  that  of  rotation,  and  consider  only  its  transla- 
tory  motion.  Therefore,  whenever  the  temperature  of  a 
quantity  of  matter  is  raised  by  any  means,  the  average 
translatory  velocity  of  its  particles  is  increased.  Now,  the 
force  of  impact  of  a  moving  body  is  proportional  to  its 
momentum,  which  is  equal  to  the  product  of  its  mass  and 
velocity  at  the  time  of  impact.  But  since  one  particle  may 
strike  another  particle  at  any  point  in  its  free  path,  here 
again  the  average  velocity  must  be  considered.  Therefore, 
since  the  mass  of  a  particle  is  a  constant  quantity,  any 
increase  in  the  average  velocity  will  cause  a  corresponding 
increase  in  momentum,  and  also  in  the  force  of  impact.  But 
the  force  of  recoil  is  practically  equal  to  the  force  of  impact, 
and  this  latter  force  is  the  repellent  force  which  tends  to 
separate  the  particles.  Thus,  with  rising  temperature  the 
repellent  force  is  increased,  the  force  of  cohesion  is  more 
and  more  nearly  overcome,  and  the  particles  become  more 
and  more  widely  separated.  Also,  with  the  rapid  decrease 
in  the  cohesive  force  incident  upon  the  increase  in  its  acting 
distance,  a  limit  is  soon  reached  beyond  which  the  force 
tending  to  cause  separation  is  greater  than  the  other,  and 
the  particles  fly  apart  indefinitely.  In  this  condition  we 
say  the  substance  is  a  gas.  If  it  was  a  liquid  or  solid  at 
the  lower  temperature,  it  has  now  been  vaporized  by  heat. 

II.       THE    THREE    STATES    OF    MATTER 

Matter  exists  in  three  states  —  the  gaseous,  the  liquid,  and 
the  solid.  In  gases  the  kinetic  energy  of  the  particles  is  so 
great  that  the  cohesive  force  is  entirely  overcome  and  the 
particles  tend  ever  to  increase  their  distance  apart.  From 
this  it  necessarily  follows  that  a  mass  of  gas  in  a  closed 


Matter  and  Its  States 


vessel  will  completely  fill  it,  no  matter  if  the  vessel  be  many 
times  the  size  of  the  original  volume  of  gas.  This  is  an 
observed  fact. 

If  such  a  gas  is  gradually  cooled  (/.  <°.,  allowed  to  do  work 
on  some  other  body  and  thus  to  part  with  some  of  the  kinetic 
energy  of  its  particles),  a  condition  will  be  reached  wherein 
the  cohesive  force  is  greater  than  the  repellent,  and  the  par- 
ticles will  remain  together  in  a  definite  volume.  As  long  as 
the  two  forces  involved  are  nearly  equal,  the  average  free 
path  will  still  be  relatively  great,  and  although  the  particles 
cling  together,  yet  they  will  move  very  freely  upon  one 
another — a  condition  imperfectly  simulated  by  the  component 
grains  in  a  mass  of  sand.  In  this  condition  the  substance 
is  said  to  be  a  liquid.  Here  the  particles  move  so  readily 
upon  one  another  that  a  mass  of  liquid  still  takes  the  form 
of  the  containing  vessel,  as  far  as  that  is  possible  without 
increase  in  volume.  In  this  regard  liquids  are  very  differ- 
ent from  gases.  Also,  oh  account  of  the  freedom  of  motion 
on  the  part  of  the  particles  making  it  up,  and  on  account  of 
the  downward  pull  of  gravity,  the  free  surface  of  a  liquid  is 
usually  approximately  level.  There  are,  indeed,  certain 
phenomena  of  surface  tension  and  adhesion  which  make  it 
possible  for  free  liquid  surfaces  to  exist  in  other  positions 
than  the  horizontal,  but  the  present  subject  does  not  lead  to 
a  discussion  of  these.  It  is  necessary  to  call  attention,  how- 
ever, to  the  fact  that,  on  account  of  the  action  of  the  cohesive 
force,  a  peculiar  surface  layer  of  particles  is  formed  about  a 
liquid  mass,  a  sort  of  thin  skin  or  film,  which  possesses  con- 
siderable tensile  strength,  and  which  is  much  less  easily 
penetrated  than  the  internal  mass. 

By  a  continuation  of  the  process  of  cooling  (which  must 
ever  be  thought  of  as  a  process  of  causing  the  body  to  give 
up  kinetic  energy  by  doing  work,  such  as  warming  another 
cooler  body)  the  liquid  particles  may  be  brought  still  closer 


8  Diffusion  and  Osmotic  Pressure 


together,  until  cohesion  becomes  so  strong,  and  hence  the 
friction  of  particle  upon  particle  so  great,  that  the  free 
movement  upon  each  other  just  described  comes  practically 
to  an  end.  The  body  is  now  a  solid  and  will  retain  its  form 
without  surrounding  walls.  The  particles  are  still  in  violent 
vibration,  however.  It  should  be  stated  here  that  the  ideal 
gas,  liquid,  or  solid  does  not  exist ;  the  hardest  substances 
show  some  tendency  to  flow  like  liquids,  and  the  most  fluid 
substances  exhibit  some  friction  of  their  component  particles 
upon  one  another. 


CHAPTER  II 
DIFFUSION  AND  DIFFUSION  TENSION 

I.       GASES 

a)  Simple  gases. — As  has  been  indicated  already,  it  is  a 
fundamental  property  of  all  gases  that  they  tend  to  fill  com- 
pletely any  vessel  in  which  they  may  be  inclosed.  Thus,  if 
a  cubic  centimeter  of  oxygen  is  measured  out  at  ordinary 
temperature  and  at  atmospheric  pressure,  and  is  then  passed 
into  a  sealed  vacuum  chamber,  it  will  completely  fill  the 
chamber,  no  matter  how  large  the  latter  may  be.  This 
process  of  expansion  is  called  diffusion.  Of  course,  in  dif- 
fusing, the  particles  of  which  the  gas  is  composed  become 
distributed  throughout  a  greater  space,  and  hence  the  gas 
becomes  less  dense.  This  property  is  often  stated  as  follows : 
"The  particles  of  gases  tend  to  separate  indefinitely.' ' 

Because  of  this  tendency  to  expand,  an  outward  pressure, 
called  gas  pressure,  is  exerted  by  a  gas  upon  the  walls  of 
any  chamber  in  which  it  may  be  confined.  Gas  pressure  is 
supposed  to  be  caused  by  the  continuous  bombardment  of 
the  walls  of  the  inclosing  vessel  by  the  vibrating  gas  par- 
ticles. If  a  gas  be  inclosed  in  a  chamber  with  elastic  walls, 
the  size  of  the  chamber  will  depend  upon  the  number  of 
particles  of  gas  present  (i.  e.,  its  concentration)  and  upon 
the  kinetic  energy  of  the  particles  themselves  (7.  <?.,  its  tem- 
perature). Thus,  for  any  temperature  and  amount  of  gas, 
the  distension  of  such  a  chamber  will  cease  when  the  inward 
pressure,  due  to  the  resilience  of  the  walls  and  to  the  pres- 
sure of  the  surrounding  atmosphere  (unless  the  chamber  be 
in  a  vacuum),  becomes  equal  to  the  outward  pressure,  due 
to  the  gas. 


10  Diffusion  and  Osmotic  Pressure 

For  a  given  amount  of  gas  the  pressure  is  constant  at  a 
constant  temperature.  But  change  in  temperature  means 
simply  change  in  the  kinetic  energy  of  the  particles.  There- 
fore a  rise  in  temperature  must  cause  a  corresponding  increase 
in  gas  pressure,  and  a  fall  a  corresponding  decrease.  Keep- 
ing the  pressure  constant,  a  rise  in  temperature  produces  an 
increase  in  volume,  and  vice  versa.  It  has  been  found 
experimentally  that  the  volume  of  a  given  mass  of  gas 
under  constant  external  pressure  varies  with  its  absolute 
temperature  (273°+  the  given  temperature  Centigrade). 
This  is  the  principle  of  Gay-Lussac,  sometimes  called 
that  of  Charles. 

But  if,  in  the  elastic  chamber  mentioned  above,  the  tem- 
perature be  kept  constant  and  the  resiliency  of  the  walls 
be  increased,  thus  increasing  the  external  pressure  on  the 
gas,  the  volume  will  be  decreased.  As  this  occurs,  how- 
ever, the  gas  will  increase  in  density,  and  continually  more 
particles  will  strike  unit  area  of  the  wall  in  unit  time.  Thus 
the  internal  pressure  upon  the  bag  will  also  be  increased, 
until  at  length  another  state  of  equilibrium  will  be  reached, 
wherein  the  external  and  internal  pressures  are  again  equal. 
But  during  the  readjustment  the  volume  of  the  gas  has 
decreased.  As  long  as  the  temperature  (i.  e.,  the  kinetic 
energy  of  the  particles)  is  constant,  an  increase  in  external 
pressure  produces  a  decrease  in  volume,  and  a  decrease  in 
external  pressure  an  increase  in  volume.  Experimentally 
it  is  demonstrated  that  the  volume  of  a  given  mass  of  gas 
at  a  constant  temperature  varies  inversely  as  the  external 
pressure  to  which  it  is  subjected.  This  is  the  principle  of 
Boyle. 

Still  another  principle  has  been  discovered  for  gases. 
If  the  volume  and  temperature  both  remain  constant,  and 
if  the  number  of  particles  is  increased  (i.  <?.,  the  concen- 
tration), the  pressure  will  be  correspondingly  increased.     It 


Diffusion  and  Diffusion  Tension  11 


is  obvious  from  the  theoretical  consideration  already  pre- 
sented that  this  must  be  true.  In  this  case  the  kinetic 
energy  of  the  particles  is  not  altered,  but  their  number 
has  been  increased,  hence  the  increase  in  pressure.  Also, 
for  a  given  concentration  and  temperature,  all  gases  exhibit 
the  same  pressure.  This  is  called  the  principle  of  Avo- 
gadro.  It  is  usually  stated  in  a  somewhat  different  way, 
namely:  Equal  volumes  of  gases,  at  equal  temperature 
and  pressure,  contain  the  same  number  of  particles.  This 
principle  holds  rigorously  true  only  for  gases  whose  con- 
centration is  rather  low.  As  a  gas  approaches  the  liquid 
state,  the  principle  of  Avogadro,  and  also  those  of  Boyle 
and  Gay-Lussac,  have  to  be  modified.  They  apply  only  to 
a  theoretically  perfect  gas. 

b)  Mixed  gases. —  In  a  mixture  of   several   gases  each 
gas  practically  exerts  its  own  pressure  independently  of  the 
others.     Thus  the  total  pressure  of  a  mixture  of  gases  in  a 
chamber  is  the  sum  of  the  pressures  which  would  be  exhib- 
ited were  the  gases  separated  and  each  put  into  a  chamber 
of  the  same  size  as  the  first  one,  the  temperature  of  course 
remaining  constant.     The  pressures  which  would   be    thus 
shown  are  called  partial  pressures,  and  the  above  fact  may 
be  stated  more  directly,  by  use  of  this  term,  as  follows:  The 
total  pressure  of  a  gas  mixture  is  practically  the  sum  of  the 
partial  pressures  of  its  component  gases.     As  a  gas  nears 
the  liquid  state,  this  principle  also  breaks  down  in  part,  it  too 
applying  rigorously  only  to  perfect  gases. 

Also,  if  two  gases  be  brought  together  so  as  to  form  two 
horizontal  strata  in  a  chamber,  diffusion  of  each  gas  will 
take  place  just  as  completely  as  if  the  other  gas  were  not 
present.  Particles  of  the  lower  gas  will  pass  up  from  the 
lower  stratum  until  that  gas  is  equally  distributed  through- 
out the  chamber.  Downward  diffusion  of  the  upper  gas  will 
occur  simultaneously,  and  the  result  of  the  two  processes 


12  Diffusion  and  Osmotic  Pressure 


will  be  a  uniform  mixture  of  the  two  gases.  If  this  pro- 
cess of  diffusion  is  obstructed  by  a  wall  placed  between 
them,  the  pressures  of  both  gases  will  of  course  be  exhibited 
independently  upon  the  opposite  sides  of  this  wall. 

II.       LIQUIDS 

a)  Simple  liquids. — When  a  liquid  is  heated,  the  kinetic 
energy  of  its  particles  is  increased,  until  at  length  the  cohe- 
sive force  which  held  them  together  is  overcome ;  then  they 
fly  off  from  the  main  mass  and  tend  ever  to  increase  their 
distance  apart.  This  is  the  process  of  vaporization  by  heat. 
As  long  as  the  temperature  remains  high  enough,  such  mat- 
ter will  remain  in  the  gaseous  state.  Also  many  substances 
which  are  usually  liquids  can  be  vaporized  at  ordinary  tem- 
peratures. Water,  alcohol,  and  ether  are  examples  of  this. 
This  process,  however,  is  a  slow  one.  It  is  explained 
theoretically  in  this  way:  Although  the  majority  of  the 
liquid  particles  cannot  break  away  from  the  main  mass  at 
ordinary  temperatures,  yet  some  of  them,  which  reach  the 
surface  with  greater  kinetic  energy  than  the  others,  do  suc- 
ceed in  breaking  through  the  firmer  surface  layer  (see  p.  7), 
and  so  escape  as  gas  particles.  If  the  chamber  above  the  liquid 
be  a  closed  one,  so  that  the  evaporated  liquid  cannot  escape, 
evaporation  soon  apparently  ceases.  If  some  of  the  liquid 
particles  come  against  the  surface  layer  with  sufficient  force  to 
pass  through  it,  it  is  reasonable  to  suppose  that,  after  escap- 
ing into  the  chamber  above,  some  of  them  may  again  pass 
through  this  film  in  the  opposite  direction,  and  so  re-enter 
the  liquid.  Here  they  come  under  the  influence  of  the  force 
of  cohesion,  which  holds  the  liquid  particles  together,  and, 
since  they  are  unable  to  break  forth  at  once,  they  remain  in 
the  liquid  state.  The  number  which  thus  re-enter  the  liquid 
will  gradually  increase  as  the  pressure  of  the  vapor  {i.  e., 
the  number  of  vapor  particles,  for  the  temperature  is  sup- 


Diffusion  and  Diffusion  Tension  13 

posedly  constant)  increases.  Thus,  an  equilibrium  will  be 
established  sooner  or  later,  wherein  the  number  of  particles 
escaping  from  the  liquid  in  unit  time  will  be  just  equaled  by 
the  number  re-entering  it.  That  is,  evaporation  is  just 
equaled  by  the  opposite  process,  condensation.  This  is  the 
condition  when  evaporation  apparently  ceases.  The  gas 
pressure  with  which  the  liquid  particles  escape  is  termed 
vapor  tension.  And  when  evaporation  has  apparently  ceased, 
the  gas  pressure  of  the  vapor  in  the  space  above  the  liquid 
is  equal  to  the  vapor  tension  which  the  particles  exhibit  in 
leaving  the  liquid  surface.  We  have  thus  a  means  for 
measuring  the  vapor  tension  of  any  liquid. 

If  the  temperature  rises,  the  vapor  tension  rises  corre- 
spondingly, following  the  principles  of  gases.  If  the 
external  pressure  upon  the  supernatant  mass  of  vapor  be 
increased,  its  gas  pressure  becomes  greater  than  the  vapor 
tension  of  the  liquid,  and  condensation  surpasses  evaporation, 
thus  decreasing  the  number  of  vapor  particles — and  hence 
the  pressure  due  to  them  —  until  equality  of  tension  and 
pressure  is  restored.  If  two  such  chambers  in  which  the 
supernatant  vapor  is  at  different  pressures  be  connected 
above  the  level  of  the  liquid,  the  substance  will  distil  over 
and  condense  in  the  chamber  which  has  the  lower  pressure. 
This  will  continue  until  the  two  pressures  have  been  equal- 
ized by  the  resulting  change  in  the  relative  volumes  occupied 
respectively  by  the  two  masses  of  liquid,  and  by  the  diffusion 
of  the  vapor  particles  themselves. 

b)  Mixed  liquids. — If  two  different,  equally  miscible 
liquids  are  brought  into  contact  with  each  other  so  as  to 
form  two  horizontal  strata,  diffusion  will  take  place  in  both 
directions,  just  as  in  the  corresponding  case  with  gases,  but 
much  more  slowly  on  account  of  the  friction  and  interfer- 
ence of  the  particles:  and  there  will  result  a  uniform  mix- 
ture in  which  both  kinds  of  particles  are  equally  distributed 


14  Diffusion  and  Osmotic  Pressure 

throughout.  This  tendency  of  one  liquid  to  diffuse  into 
another  may  be  termed  diffusion  tension;  it  corresponds  to 
the  vapor  tension  exhibited  by  an  evaporating  gas.  Diffu- 
sion in  liquids  is  sometimes  distinguished  from  that  in  gases 
by  the  use  of  the  term  "hydro-diffusion"  to  denote  the  former. 
They  are,  however,  essentially  the  same  thing.  In  the  case 
just  cited,  each  liquid  develops  a  diffusion  tension  independ- 
ently of  the  other.  Of  course,  above  such  a  mixture  of 
liquids  there  will  lie  (if  the  chamber  allow  it)  a  stratum 
of  gas  mixture  in  which  each  of  the  two  gases  has  its 
own  vapor  tension,  just  as  though  the  other  gas  were  not 

present. 

III.     solids 

a)  Simple  solids. —  Continuously  raising  the  temperature 
of  a  solid  may  result  in  liquefying  it  and  then  in  vaporizing 
the  liquid  thus  formed  ;  or  vaporization  may  take  place 
immediately,  without  the  intervention  of  the  liquid  phase 
at  all.  In  either  case  the  particles  break  away  from  the 
solid  mass  and  become  more  widely  separated.  With  the 
process  of  liquefaction,  however,  we  have  no  concern. 

When  vaporization  of  a  solid  takes  place  directly,  it  is 
called  sublimation.  Gum  camphor,  naphthalene,  and  ice 
below  the  temperature  of  melting  exhibit  this  phenomenon. 
If  the  vapor  particles  are  prevented  from  escaping,  an  equi- 
librium between  vapor  and  solid  is  ultimately  reached,  at 
which  sublimation  apparently  ceases.  At  such  a  point  sub- 
limation is  just  equaled  by  condensation.  The  whole 
process  is  analogous  to  that  of  evaporation  from  free  liquid 
surfaces.  The  pressure  of  the  vapor  surrounding  a  solid 
mass  of  the  same  substance,  when  equilibrium  is  reached, 
may  be  termed,  as  in  liquids,  vapor  tension. 

b)  Diffusion  of  two  solids. — If  two  solid  masses  of  differ- 
ent substances  are  brought  together  with  their  adjacent 
faces  in  close  contact,  there  can  be  demonstrated,  in  some 


Diffusion  and  Diffusion  Tension  15 


cases  at  least,1  a  diffusion  of  the  substances  into  each  other. 
The  process  goes  on  with  extreme  slowness,  however,  and 
the  details  need  not  be  stated  here. 

!  W-  Spring,  "Ueber  die  chemischeEinwirkungder  Korper  in  festem  Zustanda  " 
Zeitschr.f  phyaikChem.,  Vol.  II  (1888),  pp.  536-8;  Idem,  '  UebL  dls  Vorkomme'n 
gewisser  fur  den  Fltissigkeits-  oder  Gaszustand  charakteristischen  EilenschXn 
bei  festen  Metallen  »  tbia.  Vol.  XV  (1894),  pp.  65-78;  W.  Robbet8-aSstb»,  "On 
the  Diffusion  of  Gold  in  Solid  Lead  at  the  Ordinary  Temperature,"  Proceed  Aol 
Soc.  London,  Vol.  LXVII  (1901),  pp.  101-5.  rroceea.  Hoy. 


CHAPTER  III 
LIQUID  SOLUTIONS 

"  Solutions  are  homogeneous  mixtures — mixtures  which 
allow  no  separation  of  their  components  by  mechanical 
means.  The  ability  of  gases  to  form  such  mixtures  is  un- 
limited, that  of  liquids  is  limited."1  Solid  solutions  also 
exist,  but  have  not  yet  been  shown  to  play  any  part  in 
physiology;  therefore  they  need  not  be  considered  here. 
Gas  mixtures  have  already  been  discussed.  There  remains, 
then,  only  the  subject  of  liquid  solutions  —  a  very  important 
subject  in  the  study  of  physiology. 

p     SOLUTIONS    OF    LIQUIDS    IN    LIQUIDS 

Not  nearly  all  liquids  are  readily  miscible  to  form  solu- 
tions. Many  are  nearly  —  perhaps  quite  —  insoluble  in  one 
another.  Again,  many  liquids  are  mutually  soluble  in  all 
proportions  (e.  g.,  water  and  alcohol)  ;  others  are  so  only 
within  certain  limits. 

When  a  mixture  of  two  liquids  is  considered  as  a  solu- 
tion, the  liquid  which  preponderates  is  called  the  solvent 
and  the  other  the  solute.  If  such  a  solution  were  brought 
into  contact  with  a  mass  of  the  pure  solvent,  diffusion  of  the 
solute  would  take  place  into  the  pure  solvent  until  the  solute 
were  uniformly  distributed  throughout  both  layers.  At  the 
same  time  the  pure  solvent  would  diffuse  into  the  solution. 
Of  course,  the  interchange  of  particles  between  two  such 
layers  would  not  cease  when  uniformity  of  constitution  had 
been  attained  throughout;  it  would  still  go  on,  but  would 
cease  to  be  apparent,  having  become  simply  the  continuous 
motion  of  the  particles  composing  the  uniform  mixture.     At 

i  Ostwald-Walkee,  Outlines  of  General  Chemistry  (London,  1895),  p.  117. 

1G 


Liquid  Solutions  17 

the  beginning  of  such  a  process  of  mixing,  however,  a  defi- 
nite diffusion  tension  exists  and  can  be  demonstrated  —  a  dif 
fusion  tension  produced  on  the  one  hand  by  the  solute  and 
on  the  other  by  the  solvent.  These  diffusion  tensions  are 
identical  m  their  nature  with  those  spoken  of  in  the  last 
chapter.  They  increase  in  amount  with  rise  in  temperature 
and,  m  case  there  are  several  solutes,  each  one  has  its  own 
diffusion  tension.  These  facts  are  found  to  be  fundamental 
in  the  consideration  of  osmotic  pressure. 

Above  any  liquid  mixture  contained  in  a  closed  jar  which 
it  does  not  fill,  there  will  be  a  gas  mixture  of  the  vapors 
of  the  solvent  and  of  the  several  solutes.  Each  body  will 
have  its  own  vapor  pressure,  and  the  total  pressure  of  the 
gas  mixture  will  be  the  sum  of  its  partial  pressures. 

II.     solutions  of  gases  in  liquids 
Gas  solutes  behave  in  the  same  manner  as  that  just  de- 
scribed for  liquid  solutes.'     The  amount   of  gas  going  into 
solution,  when  a  mass  of  it  is  brought  into  contact  with  a 
mass  of  liquid  solvent,  increases  with  the  temperature  and 
pressure.     Diffusion   pressures    of   solvent   and   solute    are 
developed  here  also,  and  are  constant  for  a  given  tempera- 
ture ;  they  also  vary  with  the  absolute  temperature.     There 
may  be  several  gaseous  solutes  in  the  same  solution,  and  in 
this  case  each  develops  its  own  diffusion  tension  in  the  sol- 
vent.    Above  such  a  solution  there  will  be  a  gas  mixture  of 
the  vapor  of  the  solvent  and  of  the  several  solutes.     Inter- 
change of  particles  will  go  on  continually  between  the  gas 
solution  above  and  the  liquid  solution  below,  but  will  not 
be  apparent  for  reasons  similar  to  those  expressed  above  for 
liquid  solutes.     Also,  if  a  solution  containing  a  gas  solute 
be  brought  into  contact  with  a  mass  of  the  pure  solvent,  dif- 
fusion will  take  place  of  both  solvent  and  solute,  each  devel- 
oping its  own  diffusion  tension  in  its  own  direction,  just  as 


18  Diffusion  and  Osmotic  Pressure 


in  the  corresponding  case  with  a  liquid  solute.  Equilibrium 
and  an  apparent  stoppage  of  diffusion  will  be  brought  about 
when  diffusion  is  equal  in  both  directions. 

III.       SOLUTIONS   OF    SOLIDS    IN    LIQUIDS 

If  a  crystal  of  sugar  or  salt  be  put  into  the  water,  it  dis- 
solves. This  process  of  dissolving  consists  in  the  flying  off 
of  particles  into  the  water,  just  as  the  process  of  vaporiza- 
tion of  a  mass  of  naphthalene  consists  in  the  flying  off  of 
particles  into  the  air.  After  the  particles  of  the  dissolved 
substance  (solute)  are  once  free  from  the  solid  mass,  they 
behave  in  an  entirely  different  manner  from  that  which 
characterized  them  before.  While  they  were  in  the  crystal 
they  clung  together  by  cohesion.  Now  they  tend  to  sepa- 
rate as  much  as  possible  within  the  limits  of  the  solvent.1 
They  may  or  may  not  pass  the  surface  of  the  solvent  and 
enter  the  air  as  a  gas,  but  within  the  solvent  they  continue 
to  diffuse  until  they  are  uniformly  distributed.  Diffusion 
of  the  solute  in  its  solvent  takes  place  much  more  slowly 
than  does  gas  diffusion,  but  in  the  end  it  is  just  as  com- 
plete.     Thus   it    is   evident  that  within  the  volume    occu- 

i  T.  Graham,  "Ueber  die  Diffusion  von  Flussigkeiten,"  Liebigs  Ann.,  Vol. 
LXXVII  (1851),  pp.  56-89  and  129-60;  see  also  ibid.,  Vol.  LXXX  (1831),  pp.  197-201. 
The  following  references  may  serve  to  put  the  reader  into  contact  with  the 
literature  of  diffusion:  T.  Graham,  "  Supplementary  Observations  on  the  Diffusion 
of  Liquids,"  Phil.  Trans.  Roy.  Soc.  London,  1850,  pp.  805-36;  A.  FiCK,  "  Ueber  Dif- 
fusion," Pogg.  Ann.,  Vol.  XCIV  (1855),  pp.  59-86;  T.  Graham,  "  Anwendung  der  Dif- 
fusion der  Flussigkeiten  zur  Analyse,"  Liebigs  Ann.,  Vol.  CXXI  (1862),  pp.  1-77; 
F.  Voigtlaxder,  "  Ueber  die  Diffusion  in  Agargallerte,"  Zeitschr.  f.  physik.  Chem., 
Vol.  Ill  (1884),  pp.  316-35;  J.  D.  R.  Scheffer,  "  Untersuchungen  uber  die  Diffusion 
einiger  organischen  und  anorganischen  Verbindungen,"  I,  Ber.  d.  deutsch.  chem. 
Gesellsch.,  Vol.  XV  (1882),  pp.  788-801 ;  II,  ibid.,  Vol.  XVI  (1883),  pp.  1903-17 ;  W.  Nernst, 
"Zur  Kinetik  der  in  Losung  befindlichen  Korper,"  Zeitschr.  f. physik.  Chem.,  Vol.  II 
(18SS),  pp.  613-37;  J.  D.  R.  Scheffer,  "Untersuchungen  fiber  die  Diffusion  wasse- 
riger  Losungen,"  ibid.,  pp.  390-404;  S.  Arrhentus,  "Untersuchungen  uber  Diffusion 
von  in  Wasser  gelOsten  Stoffen,"  ibid.,  Vol.  X  (1892),  pp.  51-95;  L.  Liebermann  und 
S.  Burgarszky,  "  Beitrage  zur  Theorie  der  wasserigen  Losungen  von  Salzgemi- 
schen,"  ibid.,  Vol.  XII  (1893),  pp.  1S8-95;  A.  Naccari,  "Sulla  pressione  osmotica," 
Atti  delta  Reale  Accad.  dei  Lincei,  Ser.  5,  Rendiconti,  Classe  di  Scienze  fisiche, 
matemat.  e  naturali,  Vol.  II  (1893),  1  Semestre,  pp.  238-9,  and  2  Semestre,  pp.  136-8; 
R.  Hober,  "  Ueber  Concentrationsanderungen  bei  der  Diffusion  zweier  geloster 
Stoffe  gegen  einander,"  Pflugers  Archiv,  Vol.  LXXIV  (li-99),  pp.  225-45. 


Liquid  Solutions  19 


pied  by  the  solution,  dissolved  particles  exhibit  at  least  one 
of  the  fundamental  properties  of  gas  particles,  namely,  that 
of  indefinite  diffusion.  It  will  be  gathered  from  what  was 
said  under  the  preceding  headings  that  the  same  is  true  for 
liquid  and  gas  solutes. 

As  in  gases  and  in  solutions  with  liquid  and  gas  solutes, 
this  tendency  of  the  solute  to  diffuse  (diffusion  tension)  may 
be  measured.  It  is  found  that,  for  the  same  temperature 
and  volume,  the  same  number  of  particles  of  different  solutes 
gives  always  the  same  diffusion  tension.  Thus  the  solute  in 
such  a  solution  exhibits  another  principle  of  gases,  namely, 
that  of  Avogadro.  This,  too,  is  true  for  liquid  and  gas 
solutes.  The  principle  does  not  hold  rigorously  for  very 
concentrated  solutions.  There  is  developed  here  also  a  dif- 
fusion tension  on  the  part  of  the  solvent,  which  varies  with 
temperature  just  as  does  that  of  the  solute. 

If  two  solutions  containing  different  concentrations  of  the 
same  solute  in  the  same  solvent  are  brought  into  direct  con- 
tact, it  is  found  that  diffusion  of  solvent  and  solute  will  at 
length  equalize  the  concentrations  of  the  two  solutions,  so 
that  the  solute  particles  will  at  last  be  equally  distributed 
throughout  the  combined  volume.     Tjierefore_diffusion  of 
solute  particles  must  be  more  rapid  from  the  stronger  to  the 
weaker  of  the  two  solutions  than  in  the  opposite  direction. 
That  is,  the  diffusion  tension  of  the  solute  is  greater  from  a 
higher  concentration   to  a  lower  than  from    a   lower   to  a 
higher.     But  the  diffusion  tension  of  the  solvent  is  greater 
in  the  direction  from  the  lower  concentration  to  the  higher. 
This  is  also  true  in  the  case  of  gas  and  liquid  solutes.    When 
reference  is  made  to  the  "concentration"  of  a  solution,  the 
concentration  of  the  solute  is  always  meant. 


20  Diffusion  and  Osmotic  Pressure 


IV.      TERMINOLOGY    FOR    SOLUTIONS    OF    DIFFERING 

CONCENTRATION 

To  designate  different  concentrations  of  solutions,  the 
most  common  method  among  physiological  writers  has  been, 
until  quite  recently,  that  of  percentage.  An  example  of  this 
method  will  explain  its  use.  A  solution  is  said  to  be  a  5  per 
cent,  solution  of  a  certain  solute  in  a  certain  solvent  when  it 
is  composed  of  five  parts  by  weight  of  solute  to  ninety-five 
parts  by  weight  of  solvent.  But  solutions  of  different  solutes 
in  the  same  solvent  depend  for  their  physical  properties  upon 
the  relative  number  of  solute  particles  which  they  contain 
per  unit  volume.  A  glance  at  a  table  of  atomic  weights  will 
make  it  clear  that  any  method  by  weights  which  has  as  its 
basis  the  percentage  system  cannot  readily  be  adapted  to  a 
discussion  of  the  relative  number  of  molecules  contained  in 
equal  volumes  of  solutions  of  different  solutes.  Atomic 
weights,  and  therefore  molecular  weights,  cannot  readily  be 
compared  in  terms  of  percentage.  As  long  as  physiologists 
persist  in  using  this  antiquated  method  in  the  preparation  of 
their  solutions,  so  long  will  they  fail  to  arrive  at  any  far- 
reaching  principles  concerning  the  chemical  and  physical 
nature  of  the  substances  used. 

A  more  scientific  method  is  that  based  on  the  relative 
number  of  particles  of  solute  in  unit  volume  of  solution. 
We  cannot,  of  course,  actually  count  the  molecules  of  any 
substance,  but  from  a  knowledge  of  the  relative  weights  of 
the  molecules  of  different  bodies  it  is  easily  possible  to  get 
several  masses  of  different  substances,  each  of  which  will 
contain  approximately  the  same  number  of  molecules.  The 
weights  of  such  masses  must  be  to  each  other  as  the  molecu- 
lar weights  of  the  respective  substances.  For  instance,  342 
grams  of  cane  sugar  (mol.  wt.  342)  must  contain  the  same 
number  of  molecules  as  180  grams  of  glucose  (mol.  wt.  180), 
for  the  molecular  weights  give  the  relative  weights  of  the 


Liquid  Solutions  21 


two  different  molecules.  Now,  if  these  two  masses  are  placed 
in  equal  volumes  of  solution,  both  solutes  ought  to  show  the 
same  diffusion  tension.  This,  indeed,  is  found  to  be  true, 
and  the  same  principle  has  been  shown  to  be  true,  as  far 
as  experiment  has  gone,  for  solutions  of  all  substances 
which  do  not  conduct  electricity  (non-electrolytes).  Solu- 
tions of  non-electrolytes  which  contain  the  same  number 
of  molecules  per  unit  volume  have  the  same  diffusion  tension 
(at  the  same  temperature)  and  are  physically  similar.  Solu- 
tions which  conduct  electricity  exhibit  this  principle  only  in 
a  general  way.  Their  departures  from  it  and  the  reasons 
therefor  will  be  discussed  in  the  next  chapter. 

The  number  of  grams  of  a  substance  represented  by  its 
molecular  weight  is  called  a  gram-molecule.  Gram-molecules 
of  all  substances  contain,  then,  the  same  number  of  molecules. 
If  a  gram-molecule  of  some  substance  be  put  into  solution,  and 
then  this  be  diluted  to  one  liter,  there  results  a  solution  which 
can  reasonably  be  used  as  a  standard.  Such  a  solution  is  often 
termed  a  molecular  solution.  Thus,  a  molecular  solution  of 
potassium  nitrate  (mol.  wt.  101)  is  101  grams  of  the  salt  in 
a  liter  of  solution.  It  is  as  though  the  substance  had  been 
vaporized  and  the  resulting  gas  occupied  a  volume  of  one 
liter. 

But  the  analytical  chemist  has  found  it  convenient  to  use 
another  solution  as  a  standard.  He  dissolves,  to  form  a  liter 
of  solution,  as  many  grams  of  the  substance  in  question  as 
will  react  chemically  with  a  gram-molecule  of  a  monovalent 
compound.  This  amount  of  substance  is  termed  a  gram- 
equivalent.  A  gram-equivalent  of  sulphuric  acid  (H2S04) 
will  just  neutralize  a  gram-equivalent  of  potassium  hydroxide 
(KOH)  or  will  just  decompose  a  gram-equivalent  of  sodium 
chlorid  (NaCl) ;  but  it  takes  two  gram-equivalents  of  either 
of  the  last-mentioned  compounds  to  react  completely  with  a 
gram-molecule  of  sulphuric  acid.     It  follows   that  ''gram- 


22  Diffusion  and  Osmotic  Pressure 

equivalent"  and  "gram-molecule'1  are  synonymous  terms 
in  the  case  of  monovalent  compounds,  and  that  a  gram- 
equivalent  of  a  bivalent  compound  is  one  half  of  its  gram- 
molecule,  of  a  trivalent  compound  one-third,  etc. 

A  solution  made  up  so  as  to  contain  in  one  liter  a  single 
gram-equivalent  of  solute  is  termed  an  equivalent  normal, 
or  simply  a  normal,  solution.  Unfortunately  there  is  a  usage 
which  terms  a  molecular  solution  normal,  thus  giving  rise 
to  ambiguity  for  all  but  monovalent  solutes.  This  ambi- 
guity can  be  avoided  only  by  the  careful  definition  of  the 
term  "normal"  by  each  author  using  it.1  For  all  neutral 
organic  compounds,  such  as  the  sugars,  and  also  for  monova- 
lent electrolytes,  a  gram-equivalent  is  the  same  as  a  gram- 
molecule,  and  a  normal  solution  must  be  a  gram-molecule  in 
a  liter  volume.  Thus  the  sugar  solutions  described  in  a  pre- 
vious paragraph  are  both  normal  solutions.  No  ambiguity 
can  arise  from  the  use  of  the  term  in  reference  to  such  com- 
pounds. 

Regarding  acid  salts  (such  as  KHS04,  for  example),  there 
is  a  difference  of  opinion  as  to  what  should  be  denoted  by 
gram-equivalent.  Some  hold  (e.  #.,  Kahlenberg)  that  in 
the  salt  just  mentioned  gram-equivalent  and  gram-molecule 
are  identical.  Thus,  such  a  salt  might  be  regarded  as  a 
monobasic  acid.  On  the  other  hand,  Sutton,  Fresenius, 
Dandeno,  and  others  regard  a  gram-equivalent  of  KHS04 
as  one-half  a  gram-molecule.  Thus,  an  equivalent  solution 
of  this  salt  would  contain  only  one-half  gram  of  hydrogen; 
the  salt  is  to  be  regarded  as  a  monobasic  acid,  one-half  of 
whose  hydrogen  has  been  replaced.  It  seems  that  the  latter 
is  the  more  truly  scientific  position. 

1  For  an  account  of  confusion  (partly  imagined)  which  has  arisen  from  a  lack  of 
attention  to  such  definition  of  these  terms,  see  J.  B.  Dandeno,  "  The  Application  of 
Normal  Solutions  to  Biological  Problems,"  Bot.  Gaz.,  Vol.  XXXII  (1901),  pp.  229-37. 
Also  see  "Open  Letters,"  one  from  Louis  Kahlenbekg,  and  an  answer  from  Dan- 
deno, ibid.,  p.  437. 


CHAPTER  IV 

IONIZATION 
I.       IONIZATION    OF    GASES 

From  the  hypothesis  of  Avogadro  it  would  be  expected 
that,  if  a  gram-molecule  of  ammonia  and  a  gram-molecule 
of  ammonium-chlorid  vapor  were  put  into  chambers  of  the 
same  size,  the  pressures  exhibited  at  the  same  temperature  by 
the  two  gases  would  be  equal.  The  latter  substance,  how- 
ever, shows  a  far  greater  pressure.  Now,  since  the  kinetic 
theory  supposes  that  gas  pressure  is  due  to  the  kinetic 
energy  of  its  particles,  and  that  the  kinetic  energy  of  any 
particle  is  dependent  only  upon  the  temperature  to  which  it  is 
subjected,  we  must  either  reject  the  theory  when  we  come 
upon  such  a  case  as  thatjust  cited,  or  we  must  conclude  that 
there  are,  in  the  mass  of  ammonium  chlorid,  a  greater  number 
of  particles  than  in  that  of  ammonia.  Several  lines  of 
experiment  and  of  reasoning  seem  to  point  to  this  as  the 
true  condition  of  affairs.  The  number  of  molecules  is  the 
same  in  both  masses  of  gas,  but  in  the  ammonium  chlorid 
it  is  supposed  that  many  of  the  molecules  split  apart  into 
ammonia  and  hydrochloric-acid  ions  (NH3  and  HC1),  and 
that,  for  producing  pressure,  the  ions  are  as  active  as  would 
be  the  same  number  of  molecules.  In  this  way,  if  all  the 
molecules  were  dissociated,  the  pressure  should  be  twice 
that  required  by  the  theory.  The  ammonium  molecule 
seems  not  to  dissociate  at  ordinary  temperatures.  Many 
gases  exhibit  the  phenomenon  just  described;  usually  ioniza- 
tion is  not  nearly  complete  and  the  pressure  is  simply  raised 
above  its  theoretical  value.  As  the  gas  becomes  more  con- 
centrated, dissociation  becomes  less  and  less  complete. 

23 


.w  24  Diffusion  and  Osmotic  Pressure 

The  theory  just  given  is  called  the  theory  of  dissociation. 
S?  ^•^There  are  other  theories  to  account  for  these  phenomena, 
*    «Sf    but  this  has  the  widest  acceptance  at  the  present  time  and 

serves  the  purpose  of  the  physiologist  better  than  any  other 

yet  advanced. 

II.     ionization  of  solutes  in  liquid  solutions 

It  has  been  found  that  a  phenomenon  similar  to  the  one 
just  described  occurs  in  dilute  aqueous  solutions  of  electro- 
lytes. These  solutions  uniformly  give  a  higher  diffusion 
tension  than  the  one  required  by  the  theory.  The  explana- 
tion is  the  same  as  that  given  above ; 1  if  ammonium  chlorid, 
for  instance,  be  put  into  aqueous  solution,  it  has  been  shown 
that  some  of  the  molecules  ionize,  thus  increasing  the  diffusion 
tension  of  the  solute.  The  more  dilute  the  solution,  the  greater 
is  the  proportion  of  molecules  dissociated,  and  at  infinite  dilu- 
tion a  limit  would  be  reached  at  which  complete  dissociation 
would  occur.  This  theory  is  named  for  its  originator,  Arrhe- 
nius.  In  very  weak  solutions  it  is  found  that  practically  all  the 
molecules  are  ionized.  If  several  electrolytes  are  contained 
in  the  same  solution,  ionization  occurs  in  all  of  them,  but 
not  always  to  the  same  degree  as  if  there  were  but  one 
solute;  the  presence  of  other  molecules  and  ions  seems  to 
influence  the  amount  of  dissociation.  This  subject  has  not 
yet  been  sufficiently  investigated  to  permit  the  formulation 
of  a  general  principle.  Where  a  solution  contains  several 
different  kinds  of  ions,  it  is  found  that  the  velocity  of  diffu- 
sion of  some  ions  is  much  greater  than  that  of  others. 

Only  those  substances  which  conduct  electricity  when  in 
solution  are  dissociated.  The  whole  theory  of  primary  bat- 
teries and  of  electric  conduction  by  liquids  depends  upon 
this  principle.  Enough  has  been  said,  however,  to  prepare 
the  way  for  what  is  to  follow. 

1 S.  Arrhenius,  "Ueber  die  Dissociation  der  in  Wasser  gelosten  Stoffe,"  Zeit* 
schr.  f.  physik.  Chem.,  Vol.  I  (1887),  pp.  631-48. 


CHAPTER  V 

OSMOTIC  PHENOMENA 
I.    OSMOTIC    PRESSURE    OF    THE    SOLUTE 

a)  Non-electrolytes. — If  a  parchment-paper  bag  be  filled 
with  aqueous  sugar  solution  and,  after  the  opening  has 
been  sealed,  the  bag  be  submerged  in  water,  the  walls  will 
soon  be  distended  by  an  internal  pressure.  If  the  original 
solution  is  strong  enough,  the  walls  will  be  stretched  to  their 
limit  of  extensibility,  and  at  last  ruptured.  If,  during  the 
distension  of  the  bag,  the  water  around  it  be  tested,  it  will 
be  found  to  be  nearly  or  quite  free  from  sugar ;  after  the 
bag  is  ruptured,  however,  we  find  the  sugar  diffusing  rapidly 
to  the  limits  of  the  water.  Therefore  parchment  paper 
hinders  greatly  the  diffusion  of  the  sugar,  i.  e.,  it  is  only 
slightly  permeable  to  dissolved  sugar  molecules.  This  fact 
forms  the  basis  for  an  explanation  of  the  phenomenon  of 
distension  and  rupture  just  mentioned.  In  tending  to  dif- 
fuse indefinitely,  the  dissolved  molecules  (there  is  no  disso- 
ciation in  the  case  of  sugar  and  other  non-electrolytes)  bom- 
bard the  walls  of  any  chamber  in  which  they  may  be 
inclosed.1  The  fact  that  they  possess  this  property  of  in- 
definite diffusion  only  when  within  the  limits  of  the  solvent 
makes  it  necessary  that  such  a  chamber  be  surrounded  by  the 
pure  solvent,  and  that  the  solvent  permeate  its  walls.  The 
pressure  thus  produced  upon  the  walls  of  the  bag  is  reallv 
the  diffusion  tension  of  the  solute.  If  diffusion  could  take 
place  without  obstruction,  this  pressure  would  not  be  made 
apparent,  but  would  exist  none  the  less.  The  water  itself 
exerts  but  little  pressure   upon  the  walls  of  the  bag,  since 

i  J.  H.  van't  Hoff,  "  Die  Rolle  des  osmotischeu  Druckes  in  der  Analogic  zwu-chen 
LOsungen  und  Gasen,"  Zeitschr.f.  physik.  Chem.,  Vol.  I  (1887),  pp.  481-508. 

25 


26  Diffusion  and  Osmotic  Pkessure 

these  are  almost  freely  permeable  to  it,  but  a  diffusion  ten- 
sion of  water  exists  and  can  be  demonstrated  in  other  ways. 
The  internal  pressure  of  dissolved  sugar  molecules  forces  the 
walls  of  the  bag  outward  through  the  water,  just  as  a  cloth 
bag  may  be  distended  under  water  by  the  expansion  of  wire 
springs  inside  of  it.  Of  course,  in  such  a  process  of  dis- 
tension water  enters  the  bag  from  without,  the  bag  being 
permeable  to  that  substance.  If  in  either  case  the  bag  is 
not  strong  enough  to  bear  the  pressure,  it  will  burst  when  its 
limit  of  extensibility  is  reached.  If  the  parchment  bag  is 
strong  enough  to  withstand  the  pressure  developed  within, 
an  equilibrium  will  be  established,  just  as  in  the  case  of  the 
expanding  gas  described  in  chap.  ii.  In  this  condition 
of  equilibrium  the  inward  pressure  (due  to  the  resilience  of 
the  walls  of  the  bag)  is  just  equaled  by  the  outward  pres- 
sure (due  to  the  bombardment  of  the  walls  by  the  solute 
particles). 

Such  a  membrane  as  parchment  paper,  which  allows  the 
solvent  to  pass,  but  greatly  retards  or  prevents  the  passage 
of  solute  particles,  is  said  to  be  semi-permeable;  and  the 
pressure  which  such  a  membrane  makes  evident  under  the 
conditions  just  described  is  the  osmotic  pressure  of  the 
solute.  This  is  merely  the  diffusion  tension  of  the  solute, 
made  evident  by  the  opposition  of  the  membrane.  All  pos- 
sible gradations  exist  between  membranes  which  are  freely 
permeable  to  solutes  and  those  which  retard  them  or  are 
impermeable  to  them.  It  needs  to  be  noted  here,  however, 
that  a  theoretically  perfect  semi-permeable  membrane  has 
not  been  found.  The  best  ones  which  have  been  tested 
allow  some  passage  of  solute  particles.  Many  animal  mem- 
branes are  nearly  semi-permeable  in  certain  solutions,  pig's 
bladder  being  often  used.  A  membrane  of  copper  ferro- 
cyanid  is  almost  perfectly  semi-permeable  in  aqueous  sugar 
solution,  but  it  is  permeable  to  certain  salts,  e.  g.,  potassium 


Osmotic  Phenomena  27 


nitrate.     The    term  "  semi-permeable,"  therefore,    must  be 
used  with  reference  to  a  particular  solute  and  its  solvent. 

Such  membranes  as  that  of  copper  ferrocyanid  are  termed 
"precipitation membranes;"  they  are  formed  by  precipitation 
from  two  solutions  which  react  chemically.  If  a  solution 
of  potassium  ferrocyanid  and  one  of  copper  sulphate  be 
brought  together  within  the  walls  of  a  porous  clay  cup,  such 
a  membrane  (composed  of  copper  ferrocyanid,  Cu2Fe 
(CN)6)  will  be  precipitated  within  the  clay  walls.  The  mem- 
brane is  then  supported  by  the  clay,  and  the  whole  cup  may 
be  used  for  osmotic  determinations.  Another  precipitation 
membrane  is  that  formed  by  gelatin  and  tannic  acid. 

b)  Electrolytes. —  Osmotic  pressure  is  found  to  be  abnor- 
mally high  in  solutions  of  electrolytes.  This  is  one  of  the 
facts  from  which  the  conclusion  was  drawn  that  in  these 
solutions  the  diffusion  tension  of  the  solute  is  abnormally 
great,  and  hence  that  dissociation  occurs.  When  the  amount 
of  ionization  which  takes  place  in  any  solution  is  taken  into 
account,  it  is  found  that  these  solutions  are  only  apparent 
exceptions  to  the  general  rule  of  osmotic  pressure.  In  this 
case  we  can  no  longer  say  that  the  pressure  is  due  to  the 
bombardment  of  the  membrane  by  the  solute  molecules,  but 
by  the  solute  particles,  meaning  thereby  both  molecules  and 
ions.  Solutions  having  the  same  number  of  solute  particles 
per  unit  volume  have,  at  the  same  temperature,  the  same 
osmotic  pressure.  As  far  as  it  has  been  carefully  tested,  this 
principle  has  been  found  to  hold  for  all  somewhat  dilute 
solutions. 

c)  Colloids. — According  to  their  behavior  when  in  solu- 
tion, substances  have  been  classified  as  crystalloids  and  col- 
loids. Crystalloids  produce  an  osmotic  pressure  which  is 
practically  equal  quantitatively  to  the  gas  pressure  which 
would  be  produced  by  the  same  number  of  gas  particles  as 
there  are  of  solute  particles,  occupying  the  same  volume  as 


28  Diffusion  and  Osmotic  Pressure 

the  solution  and  possessing  the  same  temperature.  There 
is,  however,  a  group  of  substances  soluble  in  water,  which 
do  not  produce  osmotic  pressure  at  all,  or  produce  it  in  a 
very  slight  degree.  These  are  the  so-called  colloids,  such  as 
gelatin,  gum,  silicic  acid,  aluminium  hydroxid,  etc.  These 
substances  have  very  large  molecules  which  diffuse  with 
exceeding  slowness  and  seem  to  encounter  great  resistance 
in  passing  through  water.  Colloids  in  their  relation  to  crys- 
talloids bid  fair  to  become  very  important  in  the  advance 
of  physiological  knowledge. 

d)  Osmotic  pressure  in  general. —  Most  solutes  which 
produce  osmotic  pressure  when  in  solution  are  either  solids 
or  liquids  at  ordinary  temperatures,  when  not  in  solution. 
But  a  gas  in  solution  may  also  produce  osmotic  pressure  if 
a  suitable  membrane  is  employed. 

Osmotic  pressure  being,  in  its  origin,  perfectly  comparable 
to  gas  pressure,  the  various  principles  established  for  gas 
pressure  have  been  found  to  hold  for  osmotic  pressure.  The 
principles  of  Boyle,  of  Gay-Lussac,  and  of  Avogadro,  devel- 
oped for  gases,  have  all  been  extended  so  as  to  include  sub- 
stances in  solution.  Here  is  convincing  evidence  that  a 
solute,  as  long  as  it  is  in  solution,  is  essentially  a  gas  occu- 
pying the  volume  of  the  solution.  The  solvent  merely  pro- 
vides conditions  under  which  the  pseudo-vaporization  which 
we  call  solution  can  take  place.  Osmotic  pressure  is  inde- 
pendent of  the  solvent  and  is  dependent  only  upon  the  num- 
ber of  particles  of  solute  (i.  e.,  its  concentration)  and  upon 
their  kinetic  energy  (l  e.,  their  temperature).  The  nature 
of  the  solute  is  immaterial,  the  number  of  particles  (mole- 
cules or  ions)  per  unit  volume  being,  as  far  as  is  known,  the 
only  essential  factor. 

Where  several  substances  that  do  not  react  chemically  are 
in  very  dilute  solution,  the  osmotic  pressure  of  the  mixture 
is  the  sum  of  the  pressures  which  would  be  exhibited  were 


Osmotio  Phenomena 


'29 


each  of  the  different  solutes  dissolved  separately  to  form 
a  volume  of  solution  equal  to  the  original  volume.  As  in 
gases,  these  latter  pressures  are  termed  partial  pressures. 
This  principle  may  be  formulated  again  as  follows  :  The 
total  osmotic  pressure  of  a  dilute  solution  of  mixed  solutes  is 
the  sum  of  the  partial  osmotic  pressures  of  the  component 
solutes.  As  was  seen  in  the  last  chapter,  however,  for  more 
concentrated  solutions  this  principle  does  not  seem  to  hold. 
But  more  work  needs  to  be  done  here  before  we  may  be 
positive. 

The  principles  of  Boyle  and  Gay-Lussac  would  hold  per- 
fectly true  only  for  an  ideal  gas,  i.  e.,  one  without  any  fric- 
tion between  its  particles.  Such  a  gas  does  not  exist,  although 
hydrogen  approaches  this  condition  very  nearly.  These 
principles,  however,  hold  very  nearly  true  for  all  ordinary 
gases  as  long  as  they  are  not  nearing  the  point  of  condensa- 
tion into  a  liquid.  But,  as  has  been  stated,  in  the  vicinity 
of  this  critical  point,  whether  it  be  approached  because  of 
increase  in  pressure  or  fall  in  temperatnre,  they  do  not  hold 
true.  In  the  study  of  osmotic  pressure  it  is  found  that  a 
similar  breaking  down  of  the  same  principles  occurs  when 
the  solute  becomes  too  concentrated.  At  high  concentration 
the  principles  of  gas  pressure  no  more  apply  to  osmotic  pres- 
sure than  they  do  to  gas  pressure  itself. 

Just  what  is  the  action  of  the  membrane  in  osmotic  phe- 
nomena is  not  known.  In  many  respects  it  acts  like  a  sieve 
or  filter,  to  prevent  the  passage  of  large  particles,  but  allow 
smaller  ones  to  go  through  unhindered.  In  some  cases, 
however,  the  chemical  nature  of  the  membrane  seems  to 
come  into  play;  it  seems  to  react  chemically  with  the  solut.- 
particles,  taking  them  up  on  one  side  and  giving  them  off 
on  the  other.  But  for  a  discussion  of  the  principles  and 
general  phenomena  of  osmotic  pressure,  a  knowledge  of  the 
exact  method  by  which  the  membrane  acts  seems  not  to  be 


30  Diffusion  and  Osmotic  Pressure 


essential.  A  brief  discussion  of  the  different  theories  which 
have  been  proposed  to  account  for  the  action  of  the  membrane 
will  be  presented  in  connection  with  the  treatment  of  the 
protoplasmic  membranes  of  vegetable  cells  (see  p.  80). 

II.     diffusion  tension  of  the  solvent 

The  diffusion  tension  of  the  solvent  has  been  mentioned 
several  times  during  the  discussion  of  solutions,  but  it  is 
thought  well  to  bring  together  in  this  place  the  ideas  con- 
cerning it. 

If  the  vapor  tension  of  pure  water  be  determined  and 
then  that  of  an  aqueous  salt  or  sugar  solution,  it  will  be 
found  that  the  latter  is  invariably  less  than  the  former,  and 
this  in  proportion  to  the  concentration  of  the  solution. 
Therefore  it  must  be  that  particles  of  the  solute  hinder  the 
escape  of  the  solvent  molecules.  The  moving  particles  of 
solute  perhaps  bring  this  about  merely  by  moving  into  the 
path  of  solvent  molecules  which  would  otherwise  leave  the 
liquid.  It  is  probable  that  there  exists  also  an  attraction 
between  the  particles  of  solute  and  solvent. 

If  two  solutions  whose  concentrations  are  different  be 
brought  into  direct  contact,  as  by  placing  a  weak  sugar 
solution  over  a  stronger  one,  phenomena  similar  to  those 
just  discussed  may  be  detected.  Water  molecules  pass 
through  the  common  surface  in  both  directions.  They  are 
not  vaporized,  for  they  remain  in  the  liquid  state,  but  they 
diffuse  as  liquid  molecules.  Under  such  conditions  the  dif- 
fusion of  the  solvent  is  always  found  to  be  greater  from  the 
weaker  to  the  stronger  solution  than  in  the  opposite  direc- 
tion; it  will  be  remembered  that  thejcuost  rapid  diffusion 
of  the  solute  takes  place  from  stronger  to  weaker  solution. 
There  is,  therefore,  a  difference  in  the  energy  of  diffusion 
(diffusion  tension)  of  the  solvent  in  the  two  solutions.  This 
corresponds  to   their   difference   in  vapor   tension  just  de- 


Osmotic  Phenomena  31 


scribed.  The  diffusion  tension  of  the  solvent  is  greatest  in 
the  pure  solvent  and  decreases  as  the  concentration  of  the 
solution  increases. 

If  there  were   available   a  membrane   permeable   to  the 
solute,  but  impermeable  to  the  solvent,  this  diffusion  tension 
of  the  solvent  might  be  directly  measured.     It  would  be  an 
osmotic  pressure   similar  to  that  occasioned  by  the  solute 
molecules,    but    of    much    greater    magnitude    and    in    the 
opposite  direction.     Though   there   is  no  membrane  which 
will  make  this  pressure  evident,  most  of  the  phenomena  of 
osmotic  pressure   show  that   it  exists.     No  membrane   can 
allow  the  solvent  particles  to  pass  absolutely  without  fric- 
tion.    Thus  the  question  arises:  Why  is  not  this  pressure  of 
the  solvent  made  evident  to  a  degree  equal  to  the  amount  of 
force   needed  to  overcome  this   friction?      The   answer  is, 
obviously,  that  the  pressure  produced  by  the  solvent  on  one 
side  of  the  membrane  is  practically  equaled  by  that  on  the 
other  side.     In  solutions  where   the  principles  of  osmosis 
hold  true,  the  dilution  of  the  solvent,  due  to  the  presence  of 
the  solute,  is  negligible. 

The  following  explanation  of  osmotic  pressure  has  been 
given  by  various  authors.    The  quotation  is  from  Davenport : ! 
"Upon  the  side  containing  the  greater  number  of  molecules 
of  salt   [solute]   fewer  water    [solvent]   molecules  will  in  a 
given  time  strike  the  membrane  than  upon  the  other  side ; 
and  since  the  number  passing  through  is  proportional  to  the' 
number  striking,    relatively  fewer  molecules  of  water  will 
consequently  pass  out,  and  so  there  will  be  a  resultant  flow 
of  water  to  that  side;  and  if  the  mass  of  water  is  confined, 
it  will  exert  great  pressure."     This  explanation  is  untenable 
for  several  reasons.     Not  nearly  all  solutions  occupy  more 
space  than  the  original  mass  of  pure  solvent  from  which  they 
were  prepared.     If  to  exactly  a   liter  of  water   be  added  a 

»  C.  B.  Davenport,  Experimental  Morphology,  Vol.  I  (New  York,  1897),  p.  71. 


32  Diffusion  and  Osmotic  Pressure 


given  quantity  of  some  solute,  it  cannot  be  told  a  priori 
whether  the  resulting  solution  will  occupy  the  same  volume 
as  the  original  solvent,  or  a  greater  or  less  volume.  Into 
this  matter  it  is  unnecessary  to  go  farther  than  to  add  that 
osmotic  pressure  may  be  demonstrated  as  readily  in  solu- 
tions occupying  less  volume  than  the  original  solvent  as  in 
those  occupying  more.  It  is  obvious  that  in  the  former 
case  there  must  be  a  greater  number  of  solvent  particles 
per  unit  volume  than  in  the  pure  solvent.  Hence,  if  the 
above  explanation  can  be  retained,  there  should  be  no 
osmotic  pressure  developed  in  such  a  solution;  indeed,  it 
should  appear  on  the  side  of  the  pure  solvent. 

But  even  if  it  were  possible  that  the  entrance  of  solvent 
particles  into  the  solution  was  due  to  such  a  difference  in 
concentration  of  the  solvent  on  opposite  sides  of  the  mem- 
brane, the  explanation  just  quoted  would  fail.  It  is  incon- 
ceivable that  the  osmotic  membrane  should  be  more 
permeable  to  solvent  particles  moving  in  one  direction  than 
to  those  moving  in  the  other,  and  it  thus  becomes  impos- 
sible to  suppose  that  solvent  particles  which  can  pass  the 
membrane  in  one  direction  "will  exert  great  pressure"  upon 
it  in  the  other.  Great  hydrostatic  pressure  cannot  be  main- 
tained in  a  sieve,  nor  can  osmotic  pressure  be  maintained 
upon  a  membrane  by  a  solvent  to  which  it  is  permeable. 
Any  difference  between  the  solvent  pressures  on  the  two  sides 
of  an  osmotic  membrane  must  be  very  rapidly  destroyed  by 
diffusion  of  the  solvent  through  the  membrane. 

III.      EXPERIMENTAL    DEMONSTRATION    OF     OSMOTIC    PRESSURE 

If  a  parchment-paper  bag,  like  the  one  used  in  the  illus- 
tration of  osmotic  pressure,  were  fixed  in  a  firm  cage,  so  that 
it  could  not  expand  except  on  one  side,  and  were  then  filled 
with  solution  and  submerged  in  pure  solvent,  bulging  would 
occur  on  the  free  side.     It  would  make  no  difference  whether 


Osmotic  Phenomena  33 

this  free  side  were  serving  as  an  osmotic  membrane  or  not, 
for  pressure  produced  anywhere  in  the  bag  must  be  trans- 
mitted equally  and  undiminished  to  all  parts  of  the  surface, 
in  accordance  with  Pascal's  principle  of  transmission  of 
pressure  in  fluids.  This  transmission  would  be  accomplished 
by  the  fluid  as  a  whole,  solvent  and  solute  acting  together. 
If,  however,  the  portion  where  the  bulging  is  supposed  to 
take  place  be  permeable  to  the  solvent,  the  immediate  pres- 
sure which  affects  the  membrane  must  be  due  to  the  solute. 
Thus  it  comes  to  the  same  end  if  we  consider  the  pressure 
as  transmitted  by  the  solute,  acting  like  a  gas ;  for  increased 
energy  of  solvent  particles  will  be  transmitted  to  the  solute 
particles  with  which  they  come  in  contact. 

A  very  common  mode  of  demonstrating  the  existence  of 
osmotic  pressure  is  the  following:  A  piece  of  animal  bladder 
or  parchment  paper  is  tied  tightly  over  the  expanded  end  of 
a  thistle  tube,  and  the  bulb  is  filled  with  molasses,  or  a  strong 
aqueous  sugar  or  salt  solution.  Then  the  tube  is  fastened 
upright,  the  bulb  being  immersed  in  water  so  that  the  liquids 
within  and  without  have  a  common  level.  After  a  time  it  is 
observed  that  the  solution  has  risen  in  the  tube,  often  to  the 
height  of  a  meter  or  more  if  the  tube  is  sufficiently  long. 
The  diffusion  tension  of  the  solute  particles  within  the  bulb 
is  of  course  operative  in  every  direction,  but  osmotic  pressure 
is  developed  and  made  apparent  only  within  the  membrane. 
This  pressure  is  transmitted  through  the  liquid  to  all  parts 
of  the  surface  of  the  solution.  But  the  only  part  of  this 
surface  which  is  free  to  move,  after  the  limit  of  extensibility 
of  the  membrane  is  reached,  is  the  free  surface  in  the  stem 
of  the  tube.  We  have  seen  that  the  free  surface  of  a  liquid 
is  bounded  by  a  peculiar  layer  or  film.  Upon  this  film  the 
transmitted  osmotic  pressure  is  effective,  just  as  though  the 
film  were  a  piston  closely  fitting  within  the  bore  of  the  tube. 
In  this  case,  since  the  surface  layer  is  nearly  impermeable  to 


34  Diffusion  and  Osmotic  Pressuke 

liquid  solvent,  the  pressure  of  solvent  particles  may  be  imme- 
diately effective.  The  surface  film  is  lifted  by  the  pressure 
exerted  from  below,  as  a  piston  in  such  a  position  might  be 
lifted  by  wire  springs  situated  within  the  bulb;  and,  in 
rising,  be  the  change  in  level  ever  so  slight,  it  increases  the 
volume  of  the  solution  in  the  tube,  thus  decreasing  the  dif- 
fusion tension  of  the  solvent  within  this  solution,  and  also 
overcoming  to  some  degree  the  atmospheric  pressure  on  the 
free  surface  of  the  solution;  and  water  enters  through  the 
permeable  membrane  below.  The  entrance  of  the  water 
is  due  mainly  to  the  diffusion  tension  of  the  solvent  and  in 
part  to  hydrostatic  pressure.  In  the  latter  sense  the  push- 
ing up  of  the  surface  film  acts  like  the  raising  of  a  piston  in 
a  pump.  With  a  closed  water  manometer,  or  an  open  one 
of  mercury,  a  pressure  far  surpassing  that  of  an  atmosphere 
may  be  obtained  where  the  membrane  used  is  sufficiently 
strong.  Of  course,  in  such  cases  hydrostatic  pressure  as  a 
cause  for  the  ascent  of  the  column  is  to  be  ruled  entirely 
out  of  consideration. 

Other  methods  of  demonstrating  osmotic  pressure  are  in 
use,  but  the  explanation  just  given  may  be  applied,  mutatis 
mutandis,  to  any  of  them. 


CHAPTER  VI 

MEASUREMENT   AND    CALCULATION    OF    OSMOTIC 

PRESSURE 

I.      MEASUREMENT  OF  OSMOTIC  PRESSURE 

a)  Direct  method. — The  direct  method  of  measurement 
of  osmotic  pressure  is  very  difficult  of  operation,  and  deter- 
minations thus  made  are  exceedingly  tedious  processes. 
Nor  is  this  method  susceptible  of  sufficient  accuracy  to 
recommend  it  to  physiologists.  But  since  it  is  the  classical 
method  used  by  Pfeffer1  in  his  original  investigation  of 
the  subject,  and  since  it  has  been  used  since  that  time  by 
physical  chemists  in  establishing  the  principles  by  which 
indirect  methods  become  available,  it  will  be  described  here 
at  some  length. 

A  membrane  of  copper  ferrocyanid  is  precipitated  within 
the  walls  of  a  cup  or  bulb  of  porous  clay  (a  filter  bulb 
serves  admirably)  by  filling  the  bulb  with  a  solution  of  po- 
tassium ferrocyanid  and  surrounding  it  with  one  of  copper 
sulphate.  The  bulb  should  first  be  thoroughly  cleaned  and 
freed  from  air  by  boiling  for  some  time  in  water.  When 
the  membrane  is  well  formed  (which  occurs  after  fifteen  to 
forty  hours),  the  cup  is  filled  with  the  solution  to  be  tested, 
closed  with  a  rubber  stopper  bearing  a  mercury  manometer, 
and  immersed  in  water.  The  osmotic  pressure  rises  for  a 
number  of  hours,  being  indicated  by  the  rise  in  the  mercury 
column,  and  at  last,  when  the  membrane  has  been  ruptured 
somewhere,  begins  to  descend  again.  The  maximum  read- 
ing of  the  mercury  column  is  taken  as  the  osmotic  pressure 
of  the  solution.    The  difficulty  of  the  method  lies  in  getting 

*  W.  Pfeffer,  Osmotische  Untersuchungen,  Leipzig,  1877. 

35 


36  Diffusion  and  Osmotic  Pressure 


a  perfect  semi- permeable  membrane  which  will  withstand  the 
high  pressures  developed.  A  number  of  determinations  for 
the  same  solution  are  necessary  in  order  to  eliminate  erratic 

cases. 

The  direct  method  of  measurement  has  just  been  brought 
again  into  prominence  by  the  work  of  Morse  and  Horn.1 
These  authors  have  succeeded  in  forming  much  more  perfect 
membranes  in  porous  clay  cups  than  have  ever  been  pro- 
duced before.  Air  is  first  swept  out  of  the  pores  of  the  cup 
by  an  "endosmotic"  current.  The  cup  is  filled  with  a  weak 
solution  of  K2S04  and  immersed  in  a  vessel  of  the  same 
solution  until  the  outer  level  is  near  the  margin  of  the  cup. 
Then  a  current  from  a  dynamo  is  passed  between  a  cylindri- 
cal copper  electrode  surrounding  the  cup  and  a  platinum 
electrode  within  it.  As  the  liquid  rises  in  the  cup,  it  is 
removed,  and  in  a  short  time  the  air  is  all  removed  from  the 
porous  clay.  Then  the  cup  is  filled  with  K4Fe(CN)6  and 
immersed  as  before,  but  now  in  a  solution  of  CuS04.  The 
current  is  passed  again,  and  thus  the  Fe(CN)6  ions  are 
driven  into  the  clay  from  one  side,  while  the  Cu  ions  are 
forced  in  from  the  other.  The  resistance  of  the  cup  gradu- 
ally rises  as  the  membrane  is  formed,  being  from  fifteen 
hundred  to  three  thousand  ohms  at  the  time  when  the  mem- 
brane is  considered  as  complete.2 

b)  Indirect  methods. — Owing  to  the  difficulties  encoun- 
tered in  the  use  of  the  direct  method  just  described,  an 
indirect  method  is  usually  resorted  to.  These  indirect 
methods  depend  upon  the  general  principles,  that  depression 

iH.  N.  Morse  and  D.  W.  Horn,  "The  Preparation  of  Osmotic  Membranes  by 
Electrolysis,"  Am.  Chem.  Jour.,  Vol.  XXVI  (1901),  pp.  80-86. 

2  The  most  recent  investigations  into  the  nature  of  precipitation  membranes  are 
as  follows:  G.  Tammann,  "  Ueber  die  Permeabilitat  der  Niederschlags-Membranen," 
Zeitschr.  f.  physik.  Chem.,  Vol.  X  (1892),  pp.  255-64;  P.  Walden,  "Ueber  Diffu- 
sionserscheinungen  an  Niederschlags-Membranen,"  ibid.  (1892),  pp.  699-732;  J.  H. 
Meerburg,  "  Zur  Abhandlung  Tammanns :  Ueber  die  Permeabilitat  der  Nieder- 
schlags-Membranen," ibid.,  Vol.  XI  (1893),  pp.  446-8. 


Measurement  and  Calculation  37 


of  the  freezing-point,  elevation  of  the  boiling-point,  decrease 
of  the  vapor  tension  of  solutions,  and  osmotic  pressure  are 
all  related  phenomena,  and  may  be  obtained  one  from  the 
other  for  any  given  solution.1  Three  of  the  most  satis- 
factory methods  for  determining  osmotic  pressure  in  this 
way  will  be  briefly  described  here : 

1.  The  freezing-point  method :  The  freezing-point  of  a 
solution  is  always  lower  than  that  of  the  pure  solvent.  This 
depression  of  the  freezing-point  is  proportional  to  the  num- 
ber of  solute  particles  present,  and  therefore  to  the  osmotic 
pressure. 

The  depression  of  the  freezing-point  can  best  be  deter- 
mined by  means  of  Beckmann's  apparatus,2  which  may  be 
found  described  in  any  of  the  texts  on  physical  chemistry. 
A  determination  of  the  freezing-point  is  first  made  for 
distilled  water  ;  this  is  followed  by  a  determination  for  the 
solution  to  be  tested,  care  being  taken  not  to  disturb  the 
adjustment  of  the  thermometer  between  the  determinations. 
The  difference  between  the  two  observations  will  be  the 
required  depression,  which  may  be  denoted  by  A^.  The  rela- 
tion between  this  quantity  and  the  osmotic  pressure  is  ex- 
pressed, for  aqueous  solutions,  by  the  following  equation : 

Pf=  9173.2  A7  ,3 

wherein  Pf  is  the  osmotic  pressure  at  the  freezing-point 
of  the  solution,  measured  in  millimeters  of  mercury. 

The  osmotic  pressure  at  any  desired  temperature  other 
than  the  freezing-point,  say  T  in  the  absolute  scale,  may  be 
obtained  by  applying  the  principle  of  Gay-Lussac,  which 

1  J.  H.  van't  Hoff,  "  Die  Rolle  des  osmotischen  Druckes  in  der  Analogic 
zwischen  Losungen  und  Gasen,"  Zeitschr.f.  physlk.  Chcm.,  Vol.  I  (1S87),  pp.  481-508. 

2E.  Beckmann,  "Ueber  die  Methode  der  Molekulargewichtsbestimmuug  durch 
Gefrierpunktserniedrigung,"  ibid.,  Vol.  II  (1888),  pp.  638-45. 

3  Nernst-Palmee,  Theoretical  Chemistry  (London,  1895),  p.  132.  The  pressure 
here  is  reduced  from  atmospheres  to  millimeters  of  mercury. 


38  Diffusion  and  Osmotic  Pressure 


holds  for  osmotic  pressures  of  dilute  solutions.  This  opera- 
tion is  expressed  in  the  following: 

PTTf  =  PfT  , 

in  which  PT  is  the  osmotic  pressure,  in  millimeters  of  mer- 
cury, at  required  temperature  T  (absolute),  and  Tf  is  the 
absolute  freezing-point  of  the  solution.  From  the  equation 
we  get: 

-Ft  — ^/jT  • 

In  the  case  of  weak  aqueous  solutions,  the  freezing-point  of 
the  solution  may  be  considered,  for  this  calculation,  as  prac- 
tically the  same  as  that  of  the  solvent.  Thus  Tf  =273° 
(the  freezing-point  of  pure  water),  and  T  becomes  273  +  t, 
where  t  is  the  desired  temperature  in  the  Centigrade  scale. 
Now  the  equation  given  above  becomes: 


/(1  +  2^3')=p/(1+0-00367*) 


This  is  sufficiently  accurate  for  dilute  aqueous  solutions. 

The  freezing-point  method  is  the  simplest  and  most  satis- 
factory method  for  general  use. 

2.  The  boiling-point  method:  The  boiling-point  of  a  solu- 
tion is  always  higher  than  that  of  the  pure  solvent,  and  its 
elevation  is  proportional  to  the  osmotic  pressure  at  that  tem- 
perature. The  relation  between  the  two  quantities  for 
aqueous  solutions  is  expressed  as  follows: 

P6  =  43320A6,1 

wherein  Pb  is  the  osmotic  pressure  in  millimeters  of  mercury 
at  the  boiling-point  of  the  solution,  and  A6  is  the  elevation 
of  the  boiling-point.  The  determination  of  the  boiling- 
point  of  the  solution  and  of   distilled  water  is  best  made 

1  Neenst-Palmee,  Theoretical  Chemistry  (London,  1895),  p.  129.  The  pressure  is 
again  reduced  to  millimeters  of  mercury. 


Measurement  and  Calculation 


39 


by  Beckmann's  improved  apparatus  for  this  purpose,1  a  de- 
scription of  which  will  be  found  along  with  that  for  the 
freezing-point  determinations. 

The  correction  for  temperature  may  be  made,  as  in  the 
last  case,  by  the  application  of  the  principle  of  Gay-Lussac 
directly,  or  by  interpolation  between  Pb  and  Pf,  the  latter 
having  been  determined  by  the  previous  method. 

The  expression  for  the  Gay-Lussac  principle  is  of  course 
the  same,  mutatis  mutandis,  as  that  given  above: 


T 

■Lb 


in  which  PT  is  again  the  pressure  in  millimeters  at  the  de- 
sired temperature  T,  in  the  absolute  scale,  and  Tb  is  the 
absolute  boiling-point  of  the  solution.  For  this  calculation 
the  boiling-point  of  a  weak  aqueous  solution  may  be  con- 
sidered the  same  as  that  of  pure  water.  Thus  Tb  =  373°, 
the  boiling-point  of  water,  and  T  =  273  -f  /,  where  t  is  the' 
desired  temperature  in  the  Centigrade  scale.  Making  these 
changes  in  the  above  equation, 

273  +  * 
T~    6~373~  * 

The  method  of  interpolation  is  expressed  by  the  follow- 
ing equation: 

pt  =  Pf+~Y^Pb-pf)  , 

where  Pt  is  the  pressure  at  the  desired  temperature,  /  (Centi- 
grade), and  the  other  symbols  are  the  same  as  above. 

3.  Method  by  observed  vapor  tension:  As  has  already 
been  stated,  the  vapor  tension  of  the  solvent  is  decreased  by 
the  presence  of  a  solute.  It  is  found  that,  for  dilute  solu- 
tions, this  decrease  in  vapor  tension  is  proportional  to  the 

IB.  Beckmann,  "  Zur  Praxis  der  Bestimmung  von  Molekulargewichten  uach 
der  Siedemethode,"  Zeitschr.f.  physik.  Chem.,  Vol.  VIII  (1891),  pp.  223-8. 


40  Diffusion  and  Osmotic  Pressure 


osmotic  pressure.     This  relation  is  expressed  by  the  follow- 
ing equation: 

tt-tt'  0.0819 TX  1000s  x  760    . 


P  = 


TT 


M 


In  this  P  is  the  osmotic  pressure  in  millimeters  at  the  abso- 
lute temperature  T,  rr  and  it  '  are  the  vapor  tensions  observed 
at  that  temperature  of  the  solvent  and  solutions  respectively, 
s  is  the  specific  gravity  of  the  solution,  and  M  is  the  molecu- 
lar weight  of  the  pure  solvent.  In  the  case  of  dilute 
aqueous  solutions,  s  may  be  put  equal  to  unity  (the  specific 
gravity  of  the  pure  solvent  instead  of  that  of  the  solution), 
and  M  is  18  (the  molecular  weight  of  water) .  Making  these 
substitutions  in  the  above  equation,  we  have: 

_7r-7r/  0.0819 T  X  1000  X  760 

P_  TT  18 

or 

P  =  ^^3458r. 

IT 

The  determination  of  the  vapor  tensions  is  best  made  by 
means  of  the  method  devised  by  Ostwald  and  Walker.2 
Two  Liebig  potash  bulbs,  one  filled  with  the  solution  to  be 
tested  and  the  other  with  the  pure  solvent  (the  latter 
weighed) ,  are  joined  in  series  and  then  attached  to  a  weighed 
U-tube  of  pumice  moistened  with  sulphuric  acid.  A  slow 
current  of  air  is  passed,  for  six  to  twelve  hours,  through  the 
series.  The  air  first  becomes  saturated  at  the  tension  of  the 
solution,  and  then,  passing  through  the  second  bulb,  be- 
comes again  saturated  at  the  vapor  tension  of  the  pure 
solvent.     A  final  weighing  of  the  second  bulb  and  of  the 

i  Nernst-Palmer,  Theoretical  Chemistry  (London,  1895),  p.  126;  also  J.  H. 
VAn't  Hoff,  "Die  Rolle  des  osmotischen  Druckes  in  der  Analogie  zwischen  Losun- 
gen  und  Gasen,"  Zeitschr.f.  physik.  Chem.,  Vol.  I  (1887),  pp.  481-508. 

2  J.  Walker,  "Ueber  eine  Methode  der  Bestimmung  der  Dampfspannung  bei 
niederen  Temperaturen,"  Zeitschr.f.  physik.  Chem.,  Vol.  II  (1888),  pp.  602-5;  also 
Ostwald-Walker,  Manual  of  Physico-Chemical  Measurements  (London,  1891), 
p.  188. 


Measurement  and  Calculation  41 


sulphuric-acid  tube  gives  the  required  data.  The  amount  of 
vapor  removed  from  the  two  bulbs  respectively  is  proportional 
to  the  vapor  tensions  of  their  contents.  Thus  if  w  denote  the 
loss  in  weight  in  the  second  bulb  and  wr  the  gain  in  weight 
of  the  sulphuric  acid, 


XV  7T  —  TTf 


XV  IT 


Therefore  the  equation  given  above  may  be  written: 


W-  3458  T  . 


XV 


This  method  is  difficult  of  operation  and  not  very  satis- 
factory. The  whole  apparatus  must  be  surrounded  by  a 
jacket  to  keep  all  the  parts  at  the  same  temperature;  it  is 
not  necessary  that  the  temperature  be  absolutely  constant, 
however.  The  only  advantage  in  this  method  over  those 
previously  described  is  that  by  this  means  the  osmotic 
pressure  can  be  determined  for  the  temperature  at  which 
the  solution  is  used,  thus  avoiding  the  correction  for  tem- 
perature. 

II.     CALCULATION    OF    OSMOTIC    PRESSURE 

a)  When  the  pressure  is  produced  by  a  non-electrolyte. 
— All  solutions  of  non-electrolytes  which  contain  the  same 
number  of  molecules  per  unit  volume  of  solution  give  the 
same  osmotic  pressure.  From  measurements  made  by  Pfeffer 
we  know  that  the  osmotic  pressure  of  a  solution  of  sugar 
containing  a  gram-molecule  per  liter  is  the  same  as  the  gas 
pressure  of  a  gram-molecule  of  gas  occupying  a  liter  volume. 
This  pressure  is  22.3  atmospheres,  or  16,948  mm. of  mercury, 
at  0°  C,  or  273°  absolute.  Thus,  if  we  know  the  molecular 
weight  of  the  solute  and  the  number  of  grams  per  liter  of 
solution,  the  calculation,  on  the  principle  that  pressure  varies 
as  concentration,  is  simple  enough.  The  correction  for  tem- 
perature is  carried  out  by  the  principle  of  Gray-Lussac. 


42  Diffusion  and  Osmotic  Pressure 


For  mixed  solutions  of  non-electrolytes  the  total  osmotic 
pressure  is  the  sum  of  the  partial  pressures  due  to  the  several 
solutes  respectively. 

b)  JVhen  the  pressure  is  produced  by  an  electrolyte. — 
On  account  of  the  phenomena  of  ionization  or  dissociation, 
the  calculation  of  the  osmotic  pressure  of  a  solution  of 
an  electrolyte  becomes  somewhat  complicated.  The  amount 
of  ionization  must  be  known  in  order  to  get  the  relative 
number  of  particles  per  unit  volume.  For  instance,  a  gram- 
molecule  of  NaCl,  in  aqueous  solution,  occupying  a  liter 
volume,  contains  more  particles  than  the  same  volume  of  a 
normal  solution  of  sugar ;  some  of  the  molecules  have  sepa- 
rated into  Na  and  CI  ions. 

The  amount  of  ionization  in  any  simple  solution  of  an 
electrolyte  can  be  determined  by  means  of  the  method  of 
electrolytic  conductivity  devised  by  Kohlrausch.1  The  con- 
ductivity is  proportional  to  the  number  of  free  ions,  and 
hence,  knowing  the  conductivity  both  at  the  given  concen- 
tration and  at  a  concentration  where  ionization  is  complete, 
we  can  calculate  the  amount  of  ionization.  The  conductivity 
of  many  solutions  has  been  determined  by  different  authors 
in  different  units.  Of  course,  all  are  reducible  to  C.  G.  S. 
units  or  to  the  conductivity  of  mercury,  but  it  is  immaterial 
for  the  present  purpose  what  units  are  used,  so  long  as  the 
same  ones  are  used  throughout  the  same  calculation.  As  the 
solution  becomes  more  and  more  dilute,  the  conductivity 
approaches  a  limit.  This  limit  is  the  conductivity  at  infinite 
dilution,  where  ionization  is  complete ;  it  is  usually  denoted 
by  X^.     Allow  X  to   denote   the  conductivity  at  the  given 

concentration.     Then   —  =  a  ,    the    fraction  of    the   whole 

IF.  Kohlrausch,  Leitfaden  der  praktischen  Physik,  7  th  ed.  (Leipzig,  1892), 
p.  301 ;  also  F.  Kohlrausch  und  L.  Holborn,  Das  Leitvermdgen  der  Elektrolyte, 
Leipzig,  1898;  W.  Ostwald,  "  Ueber  Apparate  zur  Bestimmung  der  Electrischen 
Leitfahigkeit  von  Electrolyten,  Zeitschr.  f.  physik.  Chem.,  Vol.  II  (1888),  pp.  561-67; 
Ostwald- Walker,  Manual  of  Physico-Chcmical  Measurements,  London,  1S94. 


Measurement  and  Calculation  43 


number  of  molecules  which  are  dissociated.  Thus,  if  one 
out  of  every  ten  molecules  were  dissociated,  a  would  equal  T\. 
Now,  if  each  molecule  forms  k  ions,  and  if  i  denote  the 
ratio  of  the  actual  osmotic  pressure  to  that  which  would  be 
obtained  in  the  same  concentration  of  a  non-electrolyte 
solution,  i  may  be  found  from  the  following: 

t  =  l  +  (fc-l)a  . 

And  if  P  denote  the  osmotic  pressure  developed  in  a 
solution  of  a  non-electrolyte,  of  the  same  concentration  as 
that  whose  pressure  is  to  be  found,  P'  being  the  required 
pressure,  then 

P'  =  Pi  . 

The  conductivities  of  a  great  many  solutions  are  to  be 
obtained  from  published  tables.1  It  is  not  necessary  to  give 
the  methods  for  determining  these  conductivities  here. 
They  are  thoroughly  and  completely  discussed  by  Kohl- 
rausch  and  Holborn.  If  the  osmotic  pressure  is  all  that  is 
required,  and  data  for  the  conductivity  of  the  given  solute 
cannot  be  found  in  the  published  tables,  then  it  is  more  expe- 
dient to  determine  the  pressure  by  means  of  one  of  the  indirect 
methods  previously  described  than  to  determine  the  conduc- 
tivity. If  the  proper  concentration  is  not  given  in  the 
tables,  the  conductivity  for  it  is  found  by  interpolation 
between  the  conductivities  for  the  two  concentrations  nearest 
to  it.  If  the  table  is  rather  extensive  for  the  solution  in 
question,  so  that  conductivities  for  very  low  concentrations 
are  given,  it  is  usually  safe  to  take  the  highest  conductivity 
as  \M.  If  the  table  is  not  so  complete,  a  limit  for  the  con- 
ductivity has  to  be  approximated  from  the  trend  of  the  given 
data.  In  using  the  published  tables,  it  is  very  important 
that  one  bear  in  mind  the  difference  between  molecular  and 

1  W.  C.  D.  Whetham,  Solution  and  Electrolysis  (Cambridge,  189")),  pp.  218  ft.; 
also  F.  Kohlkausch  und  L.  Holborn,  Das  Leitvermogen  der  Elckrolytc,  Leipzig, 
1898. 


44  Diffusion  and  Osmotic  Pressure 

equivalent  solutions.  Most  of  the  tables  consider  as  a 
standard  solutions  containing  a  gram-equivalent  per  liter. 
These  are  easily  transformed  into  gram-molecular  solutions 
by  dividing  the  given  concentration  by  the  number  repre- 
senting the  valency  of  the  compound.  Thus  one-tenth 
gram-equivalent  per  liter  of  Na2S04  is  identical  with  one- 
twentieth  gram-molecular  solution  of  the  same  salt. 

For  very  weak  solutions  of  mixed  electrolytes  the  above 
method  may  be  resorted  to.  But  for  solutions  of  mixed 
electrolytes  and  non-electrolytes,  and  for  strong  solutions  of 
electrolytes,  no  method  of  calculation  has  yet  been  discovered. 
The  only  practical  way  open  in  such  a  case  is  to  resort  to  the 
methods  of  freezing-  and  boiling-points.  It  is  often  best  to 
make  use  of  both  these  methods,  and  to  interpolate  between 
them  for  the  normal  temperature,  inasmuch  as  ionization  often 
increases  rapidly  at  higher  temperatures.  Of  course,  where 
chemical  reaction  occurs  between  the  different  solutes,  the 
osmotic  pressure  of  the  solution  will  not  be  constant  until 
chemical  equilibrium  has  been  attained. 


PART  II 
PHYSIOLOGICAL  CONSIDERATIONS 


INTRODUCTION 

So  important  a  part  do  diffusion  and  osmotic  pressure 
seem  to  play  in  the  vital  processes  of  plants,  that  it  is  well- 
nigh  impossible  to  consider  any  phase  of  vegetable  physiology 
without  some  reference  to  these  subjects.  It  is  obviously 
not  to  the  point,  however,  to  attempt  here  a  discussion  of 
every  phenomenon  in  plant  life  into  which  they  enter. 
Rather  will  attention  be  directed  to  certain  groups  of 
phenomena  wherein  diffusion  and  osmotic  pressure  seem  to 
be  fundamental  factors.  Thus,  it  is  hoped,  may  be  formed 
a  general  conception  of  the  trend  which  modern  study  is 
taking  along  these  lines. 

Of  the  four  following  chapters,  the  first  three  have  to  do 
with  osmotic  pressure  as  an  internal  factor  in  the  life  of  the 
plant;  in  them  are  considered  the  most  important  effects  of 
the  development  of  diffusion  tensions  within  the  plant  body. 
In  the  last  chapter  are  brought  together  the  responses  of  the 
organism  to  variations  in  the  osmotic  pressure  of  the  sur- 
rounding medium.  Such  division  of  the  subject  is  merely 
expedient;  it  is  purely  artificial,  for  the  organism  and  its 
surrounding  medium  are  physically  almost  as  truly  continu- 
ous as  are  a  mass  of  ice  and  the  water  in  which  it  floats. 
Also — a  fact  which  is  often  apparently  lost  sight  of — every 
portion  of  the  plant  body  is  a  portion  of  the  environment  of 
every  other  portion.  This  is  of  fundamental  importance, 
especially  in  the  physiology  of  multicellular  forms.  How- 
ever, the  plant  body  is  a  fairly  definite  thing,  and  in  the 
present  state  of  our  knowledge  the  above  classification  of 
environmental  factors  is  perhaps  as  good  as  any  other. 

In  the  following  pages  authors  are  cited  for  the  most 

47 


48  Diffusion  and  Osmotic  Peessuee 


important  pieces  of  research,  mainly  for  the  more  recent 
ones.  References  are  not  given  for  material  which  may  be 
regarded  as  a  matter  of  common  knowledge.  To  those  who 
wish  full  citations  for  the  period  up  to  the  time  of  its  publi- 
cation, Ewart's  admirable  translation  of  Pfeffer's  Physiology 
of  Plants  will  be  found  of  great  service. 


CHAPTER  I 

TURGIDITY 

I.     PROTOPLASM   AND   ITS   LIMITING   MEMBRANES 

Anything  resembling  an  exact  knowledge  of  the  nature 
of  protoplasm  is  very  remote  from  us  as  yet,  but  we  may  be 
fairly  certain  of  this,  at  least,  that,  whatever  else  it  may  be, 
the  vital   substance  is  a  mixture  of  many  soluble  colloids 
dissolved  in,  or  impregnated  with,  an  aqueous  solution  of 
many  different  crystalloids.     Colloids  are  very  inactive  as 
far  as  diffusion  and  osmotic  pressure  are  concerned.     Thus, 
if  an  internal  diffusion  tension  is  developed  within  a  mass 
of  protoplasm,  it  must  be  mainly  due  to  the  crystalloids  dis- 
solved in  the   contained   water.     On   this   account  it  must 
come  about  that  a  mass  of  colloid  substance  inclosing  within 
its  body  an  osmotic  solution,  and  surrounded   by  another 
osmotic  solution,  will  act  somewhat  as  though  the  former 
solution  were   surrounded  by  a  semi-permeable  membrane. 
Because  of  their  slow  rate  of  diffusion,  colloid  particles  must 
in  a  measure  block  the  way  for  the  diffusion  of  crystalloid 
particles.     Hence,  if  the  more  concentrated  osmotic  solution 
be  within  the  colloid  mass,  there  will  be  developed  a  slight 
osmotic   pressure   within    the    mass,   which  will    hasten  its 
normal  process  of  swelling  by  imbibition.     With  no  truly 
semi-permeable  membrane  about  it,  no  state  of  equilibrium 
can  be  attained  between  a  colloid  body  and  the  surrounding 
medium,  until,  by  the  slow  outward  diffusion  of  the  crystal- 
loid particles  and  by  the   entrance   of   water,   there   comes 
about  a  uniform  concentration  both  within  and  without. 

In  the  author's  experiments  with  gelatin  plate  cultures  of 
Stigeoclonium  the  following  observations  were  made,  which 

49 


50  Diffusion  and  Osmotic  Pressuee 


appear  to  have  a  bearing  in  this  connection:  A  somewhat 
concentrated  solution  of  mineral  salts  was  thickened  by  the 
addition  of  enough  gelatin  to  make  a  firm  mass  at  ordinary- 
temperatures.  On  the  surface  of  this  mass  were  placed  single 
drops  of  a  dilute  solution  having  the  same  chemical  nature 
as  the  one  contained  within  the  gelatin  plate,  and  the  whole 
was  kept  in  a  moist  chamber.  After  four  or  five  hours  it 
was  always  noted  that  the  drops  of  liquid  had  disappeared; 
they  had  been  absorbed  into  the  colloid  mass.  If,  however, 
the  more  dilute  solution  were  contained  within  the  gelatin 
plate,  and  drops  of  a  concentrated  solution  were  placed  upon  its 
surface,  it  took  very  much  longer  for  total  absorption  to  occur. 
For  the  first  few  hours  there  was  usually  even  an  observable 
increase  in  the  size  of  the  liquid  drops.  Eventually  absorp- 
tion occurred,  but  it  was  often  at  the  end  of  a  period  of  more 
than  twenty-four  hours.  Of  course,  if  there  had  been  a 
semi-permeable  membrane  between  the  drops  and  the  gel- 
atin, absorption  would  not  have  taken  place.  The  gelatin 
mass  is  not  semi-permeable,  but  seems  merely  to  retard  the 
process  of  diffusion  of  crystalloid  solutes. 

If  a  mass  of  such  gelatin,  containing  a  strong  osmotic 
solution  and  surrounded  by  a  semi-permeable  membrane,  be 
placed  in  water  or  a  weaker  solution,  this  membrane  will  be 
stretched  by  the  internal  pressure  practically  as  though  no 
colloid  were  present,  and  a  state  of  equilibrium  will  be 
reached  only  when  the  resilience  of  the  membrane  equals 
the  osmotic  pressure  within.  This  has  been  demonstrated 
experimentally  by  Traube  and  Pfeffer.1  In  such  a  case  the 
osmotic  pressure  of  the  colloid  is  of  such  an  order  as  to  be 
negligible. 

Now,  any  mass  of  protoplasm  is  very  much  the  same  sort 
of  a  colloid  mass  as  the  gelatin  just  described.     Its  outer 

1M.  Traube,  "Experimente  zur  Theorie  der  Zellenbildung  und  Endosmose," 
Arch.  /.  Anat.  u.  Physiol.,  Physiol.  Abth.,  Jahrg.  1867,  pp.  87-165.  Also  Pfeffer- 
Ewart,  Physiology  of  Plants,  Cambridge,  1900,  p.  106. 


TURGIDITY  51 


layer  is  so  transformed  (perhaps  in  many  instances  by  mere 
contact  with  the  external  solution  and  with  surrounding 
objects)  that  it  is  almost  perfectly  impermeable  to  many 
solutes,  but  remains  permeable  to  water.  The  protoplast  of 
every  normal  vegetable  cell  is  thus  surrounded  by  a  more  or 
less  perfectly  semi-permeable  layer,  the  ectoplast.  If  the 
ectoplast  is  ruptured  in  any  way,  it  is  soon  re-formed,  unless 
disorganization  of  the  protoplast  ensues.1  In  such  cases 
(e.  g.,  in  Myxomycete  plasmodia,  etc.),  where  unmodified 
protoplasm  is  brought  into  contact  with  surrounding  medium, 
it  is  perhaps  partially  on  account  of  its  colloidal  nature  that 
the  contained  crystalloids  are  not  immediately  lost  by  dif- 
fusion, instead  of  being  retained,  as  they  are,  until  a  new 
surface  layer  can  be  formed. 

The  internal  osmotic  pressure,  which  results  when  the 
inclosed  solution  is  more  concentrated  than  the  external  one, 
tends  to  stretch  the  surface  layer  and  enlarge  the  protoplast. 
Against  this  pressure  is  brought  to  bear  whatever  cohesive 
and  resilient  force  the  ectoplast  may  possess;  but  this,  from 
the  semi-fluid  nature  of  protoplasm  itself,  must  be  of  a 
low  order.  In  naked  cells  this  fact  prevents  the  internal 
pressure  from  ever  becoming  very  great;  in  such  cases 
rupture  and  destruction  of  the  protoplasm  would  inevitably 
result.  But  if  the  protoplasm  is  surrounded  by  a  cellulose 
membrane,  as  in  the  case  of  the  majority  of  plant  cells,  this 
condition  is  entirely  altered;  the  swelling  of  the  protoplas- 
mic mass  is  checked  at  the  limit  of  extensibility  of  the 
inclosing  cellulose  layer.  Pressure  upon  the  ectoplast  is 
transmitted  immediately  to  the  cell  wall,  and  the  latter  is 
stretched  according  to  its  extensibility  and  to  the  pressure 
applied.  In  the  condition  of  strain  resulting  from  the 
interaction  of  the  force  of  osmotic  pressure  (the  diffusion 

1  W.  Pfeffer,  "Zur  Kenntniss  der  Plasmahaut  u.  d.  Vacuolen,"  etc.,  Abhmnll. 
d.  Jc.  sacks.  Ges.  d,  Wiss.  zu  Leipzig,  math.-physik.  Klasse,  Vol.  XVI  (1890),  pp.  187-344. 


52  Diffusion  and  Osmotic  Pressure 


tension  of  the  solute  particles)  on  the  one  hand,  and  that  of 
resilience  of  the  cellulose  membrane  on  the  other,  a  rigidity 
and  firmness  of  the  cell  as  a  whole  is  brought  about,  just  as 
a  football  or  bicycle  tire  becomes  rigid  and  firm  upon  being 
inflated  with  gas.  This  rigidity  is  termed  turgescence,  or 
turgidity.  The  term  "turgor"  has  also  been  applied  to 
this  condition,  but  it  is  better  to  reserve  this  word  to  express 
the  osmotic  pressure  of  the  internal  fluid.1  In  order  that 
the  protoplast  may  retain  its  osmotic  properties,  the  cellu- 
lose wall  must  be  permeated  with  water.  This  is  absolutely 
essential  for  the  development  of  turgidity,  since  osmotic 
pressure  is  not  active  beyond  the  limits  of  the  solvent. 
It  is,  indeed,  true  that  the  cellulose  envelope  of  every  active 
cell  is  saturated  with  water. 

Thus  far  only  those  cells  have  been  considered  which  are 
completely  filled  with  protoplasm.  This  is  the  condition  in 
young  cells,  but  mature  cells  are  not  usually  so  filled;  as 
growth  progresses,  vacuoles  of  a  watery  fluid  appear  in  the 
protoplasm.  These  increase  in  size  and  fuse  together,  until  at 
length  there  is  a  single  large  vacuole  within  the  protoplasmic 
mass.  The  typical  cell  of  plant  tissues  consists  of  a  cellulose 
wall  lined  internally  by  a  layer  of  protoplasm,  which  incloses 
a  mass  of  aqueous  solution,  the  cell  sap,  containing  sugars 
and  various  other  solutes.  The  lining  layer  of  protoplasm 
is  bounded  externally,  where  it  comes  in  contact  with  the 
cell  wall,  by  the  ectoplast.  Internally,  toward  the  vacuole, 
it  is  bounded  by  a  similar  membrane,  the  tonoplast.  The 
cellulose  wall  is  readily  permeable  to  water  and  solutes,  but 
the  protoplasmic  lining,  with  its  two  somewhat  differentiated 
limiting  layers,  normally  acts  like  a  semi-permeable  mem- 
brane, allowing  water  to  pass  quite  freely,  but  hindering,  and 
often  seeming  absolutely  to  prevent,  the  passage  of  solutes. 

1C/.  E.  B.  Copeland,  "The  Mechanism  of  Stromata,"  Ann.  Bot.,  Vol.  XVI 
(1902),  p.  330;  idem,  "The  Rise  of  the  Transpiration  Stream,"  Bot.  Gaz.,  Vol.  XXXIV 
(1902),  p.  173. 


TURGIDITY 


53 


It  is  in  these  vacuolated  cells  that  turgidity  is  developed 
to  its  greatest  extent.     It  may  be  that  continued  concentra- 
tion of  the  solution  within  the  protoplasm  itself  may  soon 
reach  a  limit  beyond  which   it  cannot  go  without  affecting 
those  energy  transformations  which  we  term  vital  activity  or 
life.     An  alteration  in  the  activities  of  the  protoplasm  thus 
produced  may  result  in  a  change  in  its  permeability  in  one 
way  or  another.     And  changes  of  this  sort  accompanied  by 
changes  in  the  chemical  activity  within  the  protoplast  may 
account  for  the  formation  of  the  vacuole  and  the  secretion  of 
osmotically  active  materials  into  it.     It  was  shown  by  Loeb1 
that  changes  in  the  concentration  of  different  ions  in  the 
protoplasm  of  animal  muscle  bring  about  marked  changes  in 
its  power  of  absorbing  water. 

At  any  rate,  however  the  vacuole  may  arise,  the  turgidity  of 
the  normal  mature  plant  cell  is  mainly  due  to  the  osmotic 
pressure  of  the  cell  sap  and  to  the  semi-permeability  of  the 
surrounding  protoplasmic  layer.     The  part   played   in  the 
development  of  turgidity  by  the  tonoplast  and  ectoplast  and  by 
the  unmodified  protoplasm  itself,  has  not  been  worked  out. 
Indeed,  the  semi-permeability  of  this  layer  can  perhaps  be 
attained  only  through  the  co-operation  of  the  three  some- 
what distinct  layers  which  make  up  the  lining  of  the  cellu- 
lose   wall.     Although  De  Vries2  was  able  to   separate    the 
tonoplast  from  the  remainder  of  the  protoplasmic  mass,  yet 
it    soon  lost  its  peculiar  properties  when    the  surrounding 
protoplasm  was  killed.   Pfeffer 3  has  shown  that  the  tonoplast 
and  ectoplast  are  equivalent  and  are  probably  formed  in  the 

i  J.  Loeb,  "On  Ion  Proteid  Compounds  and  Their  RSle  in  the  Mechanics  of  Life 
Phenomena";  I,  "The  Poisonous  Character  of  a  Pure  NaCl  Solution,"  Am.  Jour. 
Physiol,  Vol.  Ill  (1900),  pp.  327-38. 

2  Hugo  De  Vries,  u  Plasmolytische  Studien  fiber  die  Wand  der  Vacuolen," 
Jahrb.f.  wiss.  Bot,  Vol.  XVI  (1885),  pp.  465-598.  The  tonoplast  is  not  a  special  cell 
organ,  as  De  Vries  was  led  to  suppose. 

3  W.  Pfeffer,  "  Zur  Kenntniss  d.  Plasmahaut  u.  d.  Vacuolen,"  etc.,  Abhandl.  d. 
k.  sachs.  Ges.  d.  Wiss.  zu  Leipzig,  math.-physik.  Klasse,  Vol.  XVI  (1890),  pp.  187-344. 


54  Diffusion  and  Osmotic  Pressure 


same  manner.  In  consideration  of  such  facts  as  these  much 
stability  cannot  be  predicated  of  these  membranes,  and  thus, 
in  a  discnssion  of  the  osmotic  properties  of  the  cell,  it  will 
probably  be  safer  to  regard  the  intra-vacuolar  pressure  as 
arising  from  the  semi-permeability  of  the  lining  layer  of 
protoplasm  as  a  whole. 

In  a  vacuolated  cell  the  osmotic  pressure  sometimes 
becomes  so  great  as  to  burst  the  cellulose  membrane.  This 
is  notably  so  in  the  bursting  of  the  asci  in  certain  ascomy- 
cetous  fungi  and  in  the  explosion  of  the  hypo-sporangial 
region  in  Pilobolus.  Many  plant  cells  may  be  made  to  burst 
in  this  way  by  immersing  them  in  a  very  weak  solution  or 
in  distilled  water.  For  example,  Lidforss1  found  that  the 
pollen  grains  of  many  plants  (notably  the  Liliaceae,  as 
Funkia,  Asphodelus,  Anthericum,  etc.)  exploded  in  this  way 
when  put  into  water.  Noll2  has  shown  that  when  certain 
marine  Siphoneae  are  placed  in  pure  water  their  filaments 
are  apt  to  burst.  Also  Curtis3  noted  that  when  the  common 
molds  were  placed  in  water  after  having  been  accustomed 
to  a  concentrated  solution,  the  hyphal  tips  often  burst  in  the 
same  manner.  Similar  observations  were  made  among  ani- 
mals by  Gogorza,4  who  records  the  bursting  of  blood  cor- 
puscles in  certain  marine  forms  when  they  were  killed  by 
being  placed  in  fresh  water. 

II.       PLASMOLYSIS 

When  a  plant  cell  is  surrounded  by  a  solution  of  greater 
concentration  than  that  contained  within  its  vacuole,  the 
phenomenon  of  plasmolysis    occurs.     The   greater   osmotic 

i  B.Lidfoess,  M  Zur  Biologie  des  Pollens,"  Jahrb.f.  wiss.  Bot.,  Vol.  XXIX  (1896), 
pp.  1-138. 

2 F.Noll,  "Beitrag  zur  Kenntniss  der  physikalischen  Vorgange  welche  den 
Reizkrumungen  zu  Grunde  liegen,"  Arb.  d.  bot.  Inst,  in  Wiirzburg,  Vol.  Ill  (1888), 
p.  496. 

3C.   Curtis,    "Turgidity  inMycelia,"  Bull.  Torr.  Bot.  Club,  Vol.  XXVII  (1900), 

pp.  1-13. 

*  Gogorza  y  Gonzalez,  "  Influencia  del  aqua  dulce  en  los  animales  marinos," 
Annates  de  lasoc.  espagn.  hist,  nat.,  Vol.  XX  (1891),  pp.  220-71. 


TURGIDITY  55 


pressure  of  the  solutes  outside,  together  with  the  slight 
resilience  of  the  protoplasmic  layer,  cause  a  contraction  of 
the  protoplasm  resulting  in  its  separation  from  the  inclosing 
cellulose  wall.  If  the  process  of  plasmolysis  is  complete  the 
vacuole  may  disappear,  practically  all  the  water  passing  out. 
In  such  cases  the  protoplasm  often  takes  on  the  form  of  a 
solid  sphere,  which  lies  near  the  middle  of  the  cell  or  at  one 
side.  Plasmolysis  comes  about  within  a  very  few  minutes 
after  the  cell  has  been  placed  in  the  plasmolyzing  solution. 
It  is  partly  from  the  latter  fact  that  the  cellulose  wall  is 
known  to  be  permeable  to  solutes  as  well  as  to  water.  If  it  were 
not  so,  either  plasmolysis  would  not  occur,  or  the  cellulose 
membrane  would  follow  the  protoplasm  in  its  withdrawal 
toward  the  center  of  the  cell.  The  cellulose  wall  does, 
indeed,  contract  to  a  certain  measurable  extent,  but  this  is 
due  entirely  to  its  elasticity;  it  simply  returns  to  its  normal 
state  of  equilibrium  when  the  internal  pressure  of  the  turgid 
protoplasmic  sac  is  removed. 

This  fact  of  plasmolysis  has  long  been  known,1  but  the 
true  interpretation  of  it  was  due  to  De  Vries 2  and  Pfeffer.3 
After  the  relation  which  exists  between  plasmolysis,  turgidity, 
and  osmotic  pressure  was  once  established,  it  was  De  Vries* 
who  pointed  out  that  in  the  former  of  these  phenomena  we 
possess  a  means  of  measuring  the  amount  of  osmotic  pressure 
in  any  given  cell.  His  method  has  been  used  very  largely 
in  such  measurements.  It  may  be  described  as  follows: 
If  a  piece  of  plant  tissue  be  placed  in  a  concentrated  solution 

'For  a  historical  treatment  of  this  subject  see  W.  Pfeffer,  "  Zur  Kenntniss 
der  Plasmahaut  u.  d.  Vacuolen,"  etc.,  Abhandl.  d.  k.  sacks.  Ges.  d.  Wiss.  zu  Leipzig, 
math.-physik.  Klasse,  Vol.  XVI  (1890),  p.  316. 

2H.  DeVries,  Untersuchungen  uber  die  mechanischen  Ursachen  der  Zellstreck- 
ung,  Leipzig,  1877. 

3  W.  Pfeffer,  Osmotische  Untersuchungen,  Leipzig,  1877,  pp.  121  ff . 

*H.  De  Vries,  "  Eine  Methode  zur  Analyse  der  Turgorkraft,"  Jahrb.f.  wiss.  Bot., 
Vol.  XIV  (1884),  pp.  427-601.  idem,  "Osmotische  Versuche  mit  lebenden  Membranen," 
Zeitschr.f.physik.  Chem.,  Vol.  II  (1888),  pp.  415-32;  idem,  "Isotonische  Koeflizieuten 
einiger  Salze,"  ibid.,  Vol.  Ill  (1889),  pp.  103-12. 


56  Diffusion  and  Osmotic  Pressur 


E 


of  potassium  nitrate,  plasmolysis  will  occur.     If  tissues  with 
colored  cell  sap,  such  as  portions  of  the  lower  epidermis  of 
the    leaves   of   Tradescantia,   are    used,   contraction   of  the 
vacuole  may  be  seen  very  readily  under  the  microscope.    The 
coloring  matter  of  the  sap  fails   to  pass  the  protoplasmic 
layer,  and  thus  plasmolysis  is  accompanied  by  a  deepening 
of  the  color  of  the  sap.     If  the  experiment  be  repeated  on 
fresh  bits  of  tissue,  continually  weaker  and  weaker  solutions 
of  potassium  nitrate  being  used,  a  concentration  of  the  latter 
will   at    length  be  reached,   such   that    no  plasmolysis  will 
occur.     But  plasmolysis  indicates  that  the  external  solution 
is  more  concentrated  than  that  within  the  vacuole,  and  its 
failure  to  appear  indicates  that  the  cell  sap  is  more  concen- 
trated   than    the    external   solution.      Therefore,  it  may  be 
considered  that   the   maximum   concentration  of  potassium 
nitrate  which  does  not  cause  plasmolysis  is  isosmotic  (i.e., 
has  the  same  osmotic  pressure)  with  the  cell  sap.     If  we  can 
choose  a  plasmolyzing  substance  to  which  the  protoplasmic 
membrane  is  very  nearly  or  quite  impermeable1  (see  the  fol- 
lowing  section),   this   will   give    a   very  exact    method   for 
measuring  turgor  pressure.       In  this  way  De  Vries  was  able 
to  show  that,   in  general,   the  concentration  causing  plas- 
molysis was  always  the  same,  no  matter  what  substance  was 
used  to  produce  it.     There  were  some  exceptions,  however, 
glycerin  being  the  most  notable  of  those  used  by  him.     He 
found,  too,  that  certain  electrolytes  gave  extraordinarily  high 
osmotic  pressures.     The  last  is  now  known  to  be  due  to 
ionization.     The  "  isotonic  coefficients  "  given  by  this  author 
express  approximately  the  amount  of  ionization  for  the  con- 
centrations  which  he   used.      The  results   are   exceedingly 
valuable,  for  they  have  led  to  great   advance,  not  only  in 
physiology,  but  also  in  physical  chemistry ;    but  since  these 

i  De  Vries  has  given  some  very  pointed  directions  for  the  critical  use  of  his 
method  in  "Zur  plasmolytischen  Methodik,"  Bot.  Zeitg.,  Vol.  XLII  (1884)  pp. 
289-98. 


TURGIDITY  57 


coefficients  hold  true  only  within  certain  limits,  and  since 
other  more  accurate  methods  are  now  available  for  deter- 
mining the  amount  of  ionization,  a  discussion  of  them  is 
here  omitted.1 

On  the  animal  side  the  method  of  plasmolysis  has  been 
used  by  Hamburger  and  others 2  for  determining  the  osmotic 
pressures  of  the  fluid  contained  in  blood  corpuscles. 

Another  method  for  comparing  the  osmotic  pressure  of 
the  fluid  contained  in  red  blood  corpuscles  with  that  of 
the  surrounding  fluid  was  devised  by  Koppe,3  and  further 
used  by  Lob4  and  Hedin.5  The  total  volume  of  all  the  cor- 
puscles of  a  given  amount  of  blood  was  first  determined  by 
separating  them  from  the  plasma  by  means  of  the  centrifuge. 
Then  a  known  amount  of  blood  was  added  to  a  given  volume 
of  salt  solution  of  known  concentration,  and  the  mixture  was 
shaken  thoroughly.  The  corpuscles  were  then  separated 
from  the  solution  on  the  centrifuge,  and  their  total  volume 
carefully  measured.  If  the  resulting  volume  was  less  than 
the  normal  for  the  given  amount  of  blood,  the  conclusion 

i  It  will  be  well  for  physiology  when  the  practical  use  of  these  coefficients  dies 
out  entirely. 

2  H.  J.  Hamburger,  "Ueber  den  Einfluss  chemischer  Verbindungen  auf  Blut- 
korperchen in  Zusammenhang  mit  ihren  Molekulargewichten,"  Arch.  f.  Anat.  u. 
Physiol.,  Physiol.  Abth.,  Jahrg.  1886,  pp.  476-87;  idem,  "  Ueber  die  durch  Salz-  und 
Rohrzuckerlosungen  bewirkten  Veranderungen  der  Blutkorperchen,"  ibid.,  Jahrg. 
1887,  pp.  31-47;  idem,  "Die  Permeabilitat  der  rothen  Blutkorperchen  in  Zusammen- 
hang mit  den  isotonischen  Coefficienten,"  Zeitschr.  f.  Biol.,  Vol.  XXVI  (1889),  pp. 
414-33;  G.  Gryns,  "Ueber  d.  Einfluss  gelOster  Stoffe  auf  d.  rothen  Blutzellen,  in  Ver- 
bindung  mit  d.  Erscheinungen  der  Osmose  u.  Diffusion,"  Pflilgers  Arch.  f.  d.  ges. 
Physiol.,  Vol.  LXIII  (1896),  pp.  86-119;  H.  Koppe,  "Physiologische  Kochsalzlosung— 
Isotonie— osmotischer  Druck,"  ibid.,  Vol.  LXV  (1897),  pp.  492-502. 

3  H.  KOppe,  "  Eine  neue  Methode  zur  Bestimmung  isotonischer  Konzentrati- 
onen,"  Zeitschr.  f.  physik.  Chem.,  Vol.  XVI  (1895),  pp.  261-88. 

*W.  Lob,  "Ueber  Molekulargewichtsbestimmung  von  in  Wasser  loslichen  Sub- 
stanzen  mittels  der  rothen  Blutkorperchen,"  ibid.,  Vol.  XIV  (1894),  pp.  424-32. 

5S.  G.  Hedin,  "Ueber  d.  Brauchbarkeit  der  Centrifugalkraft  fur  quantitative 
Blutuntersuchungen,"  Pflilgers  Arch.  f.  d.  ges.  Physiol.,  Vol.  LX  (1895),  pp.  360- 
404;  idem,  "Ueber  d.  Bestimmung  isotonischer  Konzentrationen  durch  Zentrifagieren 
von  Blutmischungen,"  Zeitschr.  f.  physik.  Chem.,  Vol.  XVII  (1895),  pp.  164-70;  idem, 
"  Einige  Bemerkungen  Koppes  Abhandlung:  Ueber  eine  neue  Methode,  etc.,"  ibid., 
Vol.  XXI  (1896),  pp.  272-6. 


58  Diffusion  and  Osmotic  Pressure 

was  drawn  that  the  corpuscles  had  lost  water,  and  hence  that 
the  surrounding  solution  was  of  higher  osmotic  pressure 
than  the  internal  one.  Several  slight  modifications  of  the 
method  were  used,  and  many  different  solutions  were  com- 
pared, the  results  being  quite  uniform  with  those  obtained 
by  direct  observation  of  the  cells  by  DeVries  and  Hamburger. 

Still  another  manner  of  comparing  osmotic  pressures  of 
various  solutions  by  means  of  plasmolytic  phenomena  in  liv- 
ing cells  is  that  used  by  Wladimiroff,1  who  brought  motile 
bacteria  into  requisition  for  the  purpose.  He  found  that 
these  organisms  cease  to  be  motile  when  the  osmotic  pressure 
of  the  surrounding  fluid  attained  a  certain  magnitude. 
Using  as  a  criterion  the  degree  of  concentration  at  which 
motion  ceased,  he  compared  the  osmotic  pressures  of  a  num- 
ber of  solutions.  His  results  are,  in  general,  uniform  with 
those  of  the  other  authors  just  mentioned.  The  loss  of 
motion  was  due  to  extraction  of  water  in  a  manner  exactly 
analogous  to  plasmolysis. 

In  making  turgor  determinations  by  the  plasmolytic 
method  the  results  may  be  given  in  various  ways.  The 
usual  method  has  been  to  give  them  in  terms  of  a  per  cent, 
solution  of  potassium  nitrate,  sometimes  of  sodium  chlorid, 
sometimes  of  sugar,  etc.  But  with  this  method,  whenever  it 
is  desired  to  compare  pressures  which  have  been  measured 
by  means  of  different  plasmolyzing  solutions,  it  becomes 
necessary  to  make  calculations  which  involve  the  molecular 
weights  of  the  substances  used.  A  much  better  way  to 
express  turgor  pressures  is  in  terms  of  fractions  (e.  g.,  tenths) 
of  a  molecular  solution.  But  this,  although  it  suffices  for 
non-electrolytes,  fails  utterly  for  electrolytes,  because  of  the 
unequal  dissociation  of  different  compounds.  A  ^  gram- 
molecular  solution  of  KN03  will  give  a  much  greater  osmotic 

1  A.  Wladimiroff,  "  Osmotische  Versuche  an  lebenden  Bakterien,"  Zeitschr.f. 
physik.  Chem.,  Vol.VII  (1891),  pp.  529-43;  also  Zeitschr.f.  Hygien,  Vol.  X  (1891),  pp. 
89-110. 


TURGIDITY 


59 


pressure  than  a  ^  gram-molecular  solution  of  glucose.  A 
method  must  therefore  be  devised  which  will  render  it  possi- 
ble to  compare  readily  the  osmotic  pressures  of  electrolytes 
and  non-electrolytes.  This  can  be  done  by  means  of  any 
unit  of  pressure.  The  mercury  column  may  be  used,  or 
large  pressures  may  be  expressed  in  atmospheres.  Kecently 
Errera !  has  suggested  a  special  unit  for  measuring  osmotic 
pressure,  which  he  proposes  to  call  the  tonie.  It  is  to  be 
equal  to  the  pressure  of  one  dyne  upon  a  surface  of  one 
square  centimeter.  For  larger  measurements  he  suggests 
the  term  myriotonie,  equal  to  ten  thousand  tomes.  It  is 
difficult  to  see  how  this  new  unit  possesses  any  advantage 
over  the  mercury  unit  for  practical  work.  For  plasmolytic 
purposes  it  is  much  more  convenient  to  reduce  all  measure- 
ments to  terms  of  a  molecular  solution  of  a  non-electrolyte. 
Thus  comparison  becomes  easy  and  the  absolute  pressure  per 
unit  surface  can  be  readily  found  from  the  relation 

M=  22.3  atmospheres,  or  1695  cm.  Kg, 

where  M  is  the  osmotic  pressure  of  a  molecular  solution  of 
a  non-electrolyte.  Of  course,  in  making  up  solutions  of  an 
electrolyte  for  use  by  this  method  it  must  be  borne  in  mind 
that  the  desideratum  is  not  a  molecular  solution  of  the  elec- 
trolyte, but  a  solution  whose  osmotic  pressure  will  just  equal 
that  of  a  given  solution  of  a  non-electrolyte.  Thus  a  solu- 
tion of  NaCl  whose  osmotic  pressure  is,  say,  T\  M,  must 
be  considerably  more  dilute  than  a  T2F  molecular  solution 
of  that  salt. 

III.    THE    PERMEABILITY    OF    THE    PROTOPLASMIC    LAYERS 

The  often  repeated  statement  that  the  protoplasmic  layer 
is  not  permeable  to  solutes  needs  to  be  modified  as  follows: 
To  some  substances  it  is  probably  absolutely  impermeable 

1  L.  Errera,  "  Sur  la  myriotonie  comme  unite  dans  les  mesures  osmotiques," 
Extr.  des  Bull,  de  Vacad.  roy.  de  Belgique,  Vol.  Ill  (1901),  pp.  135-^3. 


60  Diffusion  and  Osmotic  Pressure 

under  certain  conditions ;  to  the  majority  of  substances  it  is 
usually  very  slightly  permeable,  but  under  certain  conditions 
its  permeability  may  increase ;  and  to  some  substances  it  is 
usually  very  readily  permeable.  Further  than  this,  the  pro- 
toplasm of  different  plants,  and  even  of  different  cells  in  the 
same  plant,  has  different  osmotic  properties.  The  condition 
of  things  is  thus  seen  to  be  very  complex.  It  will  be  of 
value  to  pass  in  review  the  most  important  fragments  of  evi- 
dence which  have  been  accumulated  upon  this  question  of 
protoplasmic  permeability. 

a)  Test  by  the  plasmolytic  method. —  There  are  several 
ways  of  testing  the  permeability  of  the  protoplasmic  sac. 
The  one  most  frequently  resorted  to  is  that  of  plasmolysis. 
A  bit  of  tissue  or  a  unicellular  organism  is  subjected  to  the 
osmotic  action  of  solutions  of  the  substance  which  is  to  be 
tested,  these  being  of  several  different  concentrations.  If 
plasmolysis  occurs  in  a  solution  of  rather  high  concentration, 
this  fact  is  taken  as  evidence  that  the  protoplasm  of  the 
given  cells  is  either  impermeable  to  the  solute  or  very 
slightly  permeable.  Of  course,  it  is  also  theoretically  pos- 
sible that  in  this  case  the  substance  used  penetrates  the 
protoplast  to  some  extent  and  causes  a  polymerization  or 
precipitation  of  the  osmotically  active  solutes  within  the  sap. 
There  is  no  evidence  for  this  phenomenon,  however,  and  its 
general  improbability  throws  it  out  of  the  category  of  seri- 
ous objections  to  the  plasmolytic  method.  If,  after  being 
left  a  short  time  in  the  plasmolyzing  solution,  the  cells 
regain  their  normal  condition,  it  shows  either  that  the  pro- 
toplasm is  somewhat  slowly  penetrated,  or  else  that  some 
osmotic  material  has  been  secreted  within  the  cell.  If  plas- 
molysis occurs  at  a  very  low  concentration,  it  is  sufficient 
proof  that  the  substance  enters  the  protoplasm;  for  such 
plasmolysis  is  due  to  alteration  in  the  membrane  through 
poisonous  action,  or  to  a  precipitation  or  some  similar  change 


TURGIDITY 


61 


within  the  vacuole,  either  of  which  phenomena  could  not  take 
place  without  penetration.  If  plasmolysis  does  not  occur 
even  at  high  concentrations,  we  have  evidence  that  the  pro- 
toplasmic sac  is  not  only  penetrable  to  the  substance  used, 
but  that  this  substance  has  no  marked  immediate  toxic 
action. 

Cane  sugar,  glucose,  KN03,  and  NaCl  are  usually  found 
to  produce  permanent  plasmolysis.  Plant  cells  placed  in 
concentrated  solutions  of  these  substances  do  not,  as  a  rule 
regain  their  original  turgid  condition  as  long  as  they  remain 
therein  ;  no  perceptible  penetration  occurs.  However,  there 
are  many  cells  whose  protoplasts  are  more  or  less  permeable 
to  these  compounds,  and  there  are  all  gradations  between 
absolute  impermeability  and  rather  slow  permeability.  One 
extreme  of  this  series  is  Massart's  Bacterium  termo,1  which 
was  not  plasmolyzed  at  all  in  strong  solutions  of  cane  su-ar 
and  KN03. 

But  in  most  cases  plasmolysis  is  the  first  result  of  irri- 
gating the  cells  with  the  test  solution,  and  it  is  only  after 
the  lapse  of  some  time  that  the  first  effect  disappears.     The 
gradual  inward  diffusion  of  the  external  osmotic  substance, 
or,  in  some  cases,  the  gradual  secretion  of  an  osmotic  substance 
within  the  cells,  finally  brings  about  an  equalization  of  the 
internal  and  external  pressures.     Then  the  original  internal 
pressure,    produced    by    the    solutes    within    the    vacuole, 
becomes  again  effective  in  producing  turgidity.     De  Vries ' 
found  that  the  tonoplasts  of  various  plant  cells  were  gener- 
ally freely  penetrated  by  acids  and  alkalies,  but  that  salts 
passed    these    membranes   much    more   slowly.      However, 
many  cells  were  found  which,  after  being  plasmolyzed  in  a 
solution  of  KN03  or  NaCl,  gradually  returned  to  their  origi- 

1  Massaet,  "Sensibility  et  adaption  des  organismes   a   la  concentration   des 
solutions  salines,"  Arch,  de  biol.,  Vol.  IX  (1899),pp.  515-70. 

2H.  De  Vries,  "  Plasmolytische  Studien  uber  die  Wand  der  Vacuolen  "  Jahrb 
f.  wiss.  Bot.,  Vol  XVI   (1885),  pp.  465-598. 


62  Diffusion  and  Osmotic  Pressure 


nal  condition  if  left  in  the  plasmolyzing  solution.  This 
author  also  observed  that  the  presence  of  an  acid  or  base,  or 
of  any  other  poisonous  substance,  made  the  protoplasm 
rapidly  permeable  to  such  salts  as  KN03  and  NaCl.  The 
cells  of  the  epidermis  of  leaves  of  Tradeacantia,  Curcuma, 
and  Begonia  rex  appeared  to  be  impermeable  to  KN03. 
The  same  author1  found  the  protoplasm  of  beets  to  be  per- 
meable to  NaCl.  Janse2  found  a  similar  return  of  tur- 
gidity  in  the  case  of  marine  algse  (e.  g.,  Chsetomorpha) 
which  were  allowed  to  remain  in  a  solution  of  KN03  or  of 
NaCl  which  plasmolyzed  them  at  first.  He  also  found  that 
the  protoplasts  of  these  algae  are  permeable  to  cane  sugar. 
When  plasmolysis  was  brought  about  in  a  solution  of  this 
substance,  turgor  gradually  returned,  but  this  process  took 
about  four  times  as  long  here  as  in  a  KN03  solution.  In 
Spirogyra  the  same  general  facts  were  observed,  but  the 
permeability  is  not  as  marked  here  as  in  the  marine  forms. 
In  summing  up  the  results  of  his  second  paper,  this  author 
states  that  he  has  found  the  protoplasm  of  the  following  five 
plants  permeable  as  follows  : 

Chsetomorpha  is  permeable  to  KN03,  NaCl,  cane  sugar. 
Spirogyra  is  permeable  to  KN03,  NaCl,  grape  sugar. 
Tradescantia  and  Curcuma  are  permeable  to  KN03,  NaCl. 
Stratiotes  is  permeable  to  KN03. 

Glycerin  and  urea  have  been  shown  by  De  Vries3  and 
Klebs 4  to  penetrate  nearly  all  plant  cells  with  great  readi- 

i  H.  De  Vkies,  "Sur  la  permeability  du  protoplasma  des  better^yes  rouges," 
Arch.  n6erl.,  Vol.  VI  (1871),  pp.  117-26. 

2  J.  M.  Janse,  "  Plasmolytische  Versuche  an  Algen,"  Bot.  Centralbl.,  Vol.  XXXII 
(1887),  pp.  21-6;  idem,  "Die  Permeabilitat  des  Protoplasma,"  Verslag.  en  Mededeel. 
d.  k.  Akad.  v.  Wetensch.  te  Amsterdam,  3  Reihe,  Vol.  IV  (1888),  p.  332. 

3H.  De  Vries,  "Ueber  den  isotonischen  Coefficient  des  Glycerins,"  Bot.  Zeitg., 
Vol.  XLVI  (1888),  pp.  229  ff.;  idem,  "Ueber  die  Permeabilitat  der  Protoplaste  fur 
Harnstoff,"  ibid.,  Vol.  XLVII  (1889),  pp.  309  ff. 

*G.  Klebs,  "Beitrage  zur  Physiologie  der  Pflanzenzelle,"  Unters.  aus  d.  bot. 
Inst,  zu  Tubingen,  Vol.  II  (1888),  pp.  489  ff.;  idem,  "  Beitrage  zur  Physiologie  der 
Pflanzenzelle,"  Ber.  d.  deutsch.  bot.  Ges.,  Vol.  V  (1887),  pp.  181-9. 


TlJRGIDITY  63 


ness.  Plasmolysis  occurs,  but  is  of  short  duration  ;  Overton1 
found  that  it  took  from  two  to  five  hours  for  equilibrium  to 
be  re-established  in  solutions  of  these  substances. 

There  are  exceptions  here  also,  however,  for  De  Vries 
found  that  the  cells  of  the  bud  scales  of  Begonia  manicata 
were  almost  impermeable  to  glycerin  and  urea.  Jennings2 
states  that  paramoecia  are  permanently  plasmolyzed  in 
glycerin. 

By  an  extensive  investigation  of  the  plasmolytic  behavior 
of  various  organic  compounds  Overton3  found  that  a  great 
number  of  these  do  not  produce  plasmolysis  at  all,  so  rapidly 
do  they  penetrate  the  protoplasm.  Among  these  substances 
may  be  named  :  ethyl  alcohol,  ethyl  ether,  formaldehyde, 
chloral  hydrate,  acetone,  methyl  cyanid,  furfurol,  caffein, 
etc.  The  list  includes  practically  all  of  the  aliphatic  alco- 
hols and  related  compounds  which  are  soluble  in  water,  and 
also  a  number  of  soluble  aromatic  compounds,  such  as 
anilin,  acetanilid,  phenol,  phloroglucin,  etc.  Although  the 
author  does  not  express  himself  on  this  point,  it  seems 
probable  that  the  apparent  plasmolysis  which  occurs  when 
plant  cells  are  placed  in  strong  alcohol  is  due,  not  to  osmotic 
pressure,  but  to  an  increased  permeability  in  the  osmotic 
membranes  (due  to  the  poison)  and  also  to  an  active  con- 
traction on  the  part  of  the  protoplasm.  The  same  author 
shows  that  there  are  all  gradations  in  rapidity  of  penetra- 
tion, from  those  substances  which  fail  to  plasmolyze  at  all 
to  those  which  produce  permanent  plasmolysis.  In  all  these 
cases  he  found  that  the  protoplasmic  sac  is  as  readily  per- 
meated outward  as  inward.      Practically  all  liquids  which 

i  E.  Overton,  "  Ueber  die  osmotischen  Eigenschaften  der  lebenden  Pflanzen- 
und  Tierzelle,"  Vierteljahrschr.  d.Naturf.-Ges.  in  Zurich,  Vol.  XL  (1895),  pp.  159-84. 

2  H.  S.  Jennings,  "  Studies  on  the  Reactions  to  Stimuli  in  Unicellular  Organ- 
isms": I,  "Reactions  to  Chemical,  Osmotic,  and  Mechanical  Stimuli  in  the  Ciliate 
Infusoria,"  Jour.  Physiol.,  Vol.  XXI  (1897),  pp.  258-322. 

3  E.  Oveeton,  loc.  cit.  The  table  of  compounds  occurs  on  p.  181. 


64  Diffusion  and  Osmotic  Pressure 

are  soluble  in  water  penetrate  readily,  glycerin  being  one 
of  the  slowest.  It  appears  as  if  the  power  to  penetrate 
decreased  with  increasing  specific  gravity.  Overton  finds 
that  the  same  thing  is  generally  true  of  animal  cells  also, 
and — what  is  still  more  striking — that  the  amount  of 
alcohols,  etc.,  which  plant  and  animal  cells  are  able  to  bear 
is  nearly  the  same. 

When  the  solute  fails  to  penetrate  the  protoplast,  the 
osmotic  concentration  necessary  for  plasmolysis  is  constant, 
no  matter  what  the  solute  may  be.  But  the  more  readily  it 
penetrates,  the  higher  the  concentration  necessary  to  bring 
about  plasmolysis,  until  at  last,  as  in  the  alcohols  and  ethers, 
this  phenomenon  does  not  truly  occur  at  all. 

b)  Direct  test  of  penetrability. — Another  method  of 
determining  the  extent  of  permeability  manifested  by  pro- 
toplasm is  to  identify  the  diffusing  substance  after  it  has 
passed  the  plasmic  layer.  De  Vries1  showed  that  the  pene- 
tration of  dilute  ammonia  into  the  cells  of  the  red  beet  can 
be  demonstrated  by  the  reaction  of  the  colored  cell  sap  to 
this  substance.  The  red  sap  changes  to  blue  upon  contact 
with  an  alkali.  By  choosing  other  cells  whose  sap  contains 
red  and  blue  dissolved  pigments,  Pfeffer2  showed  that  not 
only  ammonia,  but  also  the  caustic  alkalies  and  alkaline  car- 
bonates as  well  as  acids  (such  as  tartaric,  phosphoric,  and 
carbonic)  pass  very  rapidly  through  plant  protoplasm.  We 
may  consider  that  this  at  least  proves  that  the  H  and  OH 
ions  penetrate.  In  some  cases  a  precipitate  may  be  produced 
within  the  vacuole  by  the  reaction  of  the  penetrating  sub- 
stance with  the  materials  of  the  cell  sap.  This  is  the  case 
with  caffein,  antipyrin,  and  some  others.3 

i  H.  De  Vries,  "  Sur  la  permeabilite  du  protoplasme  des  betteraves  rouges," 
Arch.  n£erl,  Vol.  VI  (1871),  p.  124;  idem,  "Sur  la  mort  des  cellules  veg6tales," 
ibid.  (1871),  pp.  245-95. 

2W.  Pfeffer,  Osmotische  Untersuchungen,  Leipzig,  1877,  p.  140. 

3  Pfeffer-Ewart,  Physiology  of  Plants,  1900,  p.  98. 


TURGIDITY  65 


Another  manner  of  carrying  out  the  direct  test  is  to  place 
cells  in  the  solution  to  be  tested  and,  after  sufficient  time  has 
elapsed,  to  treat  them  with  a  reagent  which  will  penetrate  * 
and  also  give  a  visible  reaction  with  the  substance  to  be 
tested.  Thus,  if  penetration  took  place  in  the  first  solution 
a  microchemical  test  for  the  solute  should  be  obtained  within 
the  vacuole.  In  this  way  diphenylamin  was  first  used  by 
Molisch1  to  detect  KN03  in  plant  cells.  With  this  reagent 
a  nitrate  is  indicated  by  the  appearance  of  a  blue  color.  It 
appears  from  the  work  of  Molisch,  Wieler,  De  Vries,  and 
Janse,  in  which  this  method  was  used,  that  plant  protoplasm  is 
very  generally  penetrated  by  KN03  in  dilute  solution.  In  a 
similar  manner  tests  have  been  made  with  Fehling's  solution, 
which  prove  the  penetration  of  glucose. 

Janse 2  showed  in  this  manner  that  Spirogyra  protoplasts 
are  penetrated  by  KN03.  Wieler3  worked  with  entire 
plants  of  the  angiosperm  group  and  obtained  similar  results. 
By  means  of  diphenylamin  and  sulphuric  acid  he  was  able  to 
demonstrate  that  N03  ions  penetrate  the  protoplasts  of  seed- 
ling beans,  sunflowers,  etc.  By  platinum  chlorid  he  also 
demonstrated  the  penetration  of  K  ions.  Stems  placed  in  a 
sugar  solution  formed  starch,  while  a  control  without  sugar 
failed  to  do  so.  It  made  no  difference  whether  cane  sugar 
or  glucose  were  used.  This  proves  the  power  of  these  sugars 
to  penetrate  the  protoplasts  in  stems.  The  last-named 
author  also  presents  evidence  that  the  roots  of  seedlings  of 
Vicia  faba  are  able  to  absorb  glucose  from  a  solution.     Of 

i  H.  Molisch,  "Ueber  den  mikrochemischen  Nachweiss  von  Nitraten  und 
Nitriten  in  der  Pflanze  mittelst  Diphenylamin  oder  Brucin,"  Ber.  d.  deutsch,  bot 
Ges.,  Vol.  I  (1883),  pp.  150-55;  also  Sitzungsber.  d.  kais.  Akad.d.  Wiss.  zu  Wien,  math.- 
nat.  hist.  Klasse,  Vol.  I  (1887),  p.  221. 

2  J.  M.  Janse,  "Plasmolytische  Versuche  an  Algen,"  Bot.  Centralbl..  Vol.  XXXII 
(1887),  pp.  21-6;  idem,  "Die  Permeabilitat  des  Protoplasma,"  Verslag.  en  Mcdcdeel. 
d.  k.  Akad.  v.  Wetensch.  te  Amsterdam,  3  Reihe,  Vol.  IV  (1888),  p.  332. 

3  A.  Wieler,  "  Plasmolytische  Versuche  mit  unverletzten  Phanerogamen,"  Ber. 
d.  deutsch.  bot.  Ges.,  Vol.  V  (1887),  pp.  375-80. 


66  Diffusion  and  Osmotic  Pressure 


course  it  is  possible  that,  in  these  cases,  the  substance  in 
question  does  not  pass  the  protoplasm  as  such,  but  is  modified 
at  the  surface  of  the  ectoplast  and  penetrates  in  another 
form.  For  this  question  there  seems  to  be  as  yet  no  method 
of  attack.  Wortmann1  believed  he  had  evidence  that  the 
starch  in  the  endosperm  of  seeds  was  not  acted  upon  by  an 
enzyme  but  by  the  protoplasm  itself ;  this,  however,  has  been 
disproved.2  Since  protoplasm  contains  so  many  enzymes  of 
one  sort  and  another  it  seems  impossible  to  gather  evidence 
as  to  whether  a  given  action  takes  place  within  the  proto- 
plasmic mass  or  outside  of  it.  It  is  probable  that  it  occurs 
wherever  the  enzymes  are  present,  whether  this  be  within 

or  without. 

The  penetration  of  anilin  dyes  has  been  studied  exten- 
sively by  Pfeffer,3  who  showed,  for  example,  that  the  sap  of 
living  cells  may  be  strongly  stained  by  the  inward  diffusion 
of  methyl  blue,  methyl  violet,  etc.  Certain  anilin  dyes  may 
be  absorbed  into  the  living  protoplasm  itself  and  held  there 
so  as  to  give  a  marked  stain.  Even  the  nucleus  may  be  so 
stained  while  living  by  means  of  dahlia,  mauvein,  etc.* 

In  most  of  these  experiments  there  is  an  accumulation  of 
the  stain  in  the  vacuole  or  within  the  protoplasm.  Thus,  if 
plants  of  Elodea  canadensis  be  placed  for  several  days  in  a 
weak  solution  of  methyl  blue,  they  become  visibly  stained, 
while  the  external  solution  loses  its  color.  Examination 
shows  that  the  protoplasm  itself  is  not  colored,  but  that  the 

i  J.  Woktmann,  "  Ueber  den  Nachweiss,  das  Vorkommen  und  die  Bedeutung  des 
diastatischen  Enzyms  in  den  Pflanzen,"  Bot.  Zeitg.,  Vol.  XLVIII  (1890),  pp.  581  ff. 

2B.  Hansteen,  "  Ueber  die  Ursachen  der  Entleerung  der  Reservestoffe  aus 
Samen,"  Flora,  Vol.  LXXIX  (1894),  pp.  419-29. 

3Pfeffer-Ewart,  Physiology  of  Plants,  Cambridge,  1900,  p.  96.  References  to  an 
extensive  literature  are  there  given,  the  most  important  of  which  is :  W.  Pfeffek, 
"Ueber  Aufnahme  von  Anilinfarben  in  lebenden  Zellen,"  Unters.ausd.  bot.  Inst,  zu 
Tubingen,  Vol.  II  (1886),  pp.  179-331 ;  see  also  E.  Ovekton,  "  Studien  fiber  d.  Aufnahme 
der  Anilinfarben  durch  die  lebende  Zelle,"  Jahrb.  f.  wiss.  Bot,  Vol.  XXXIV  (1900), 
pp.  669-701. 

4  D.  H.  Campbell,  "  The  Staining  of  Living  Nuclei,"  Unters.  aus  d.  bot.  Inst,  zu 
Tiibingen,  Vol.  II  (1888),  pp.  569-81. 


TURGIDITY  67 


dye  has  accumulated  in  the  vacuole,  there  becoming  much 
more  concentrated  than  was  the  original  external  solution. 
This  phenomenon  will  be  discussed  under  e). 

Another  method  by  which  direct  determinations  of  per- 
meability may  be  made  is  to  analyze  the  plant  or  its  juice 
after  the  culture  has  been  grown  some  time  in  a  medium  of 
known  content.  In  this  way  von  Mayenburg1  found  that, 
out  of  a  series  of  substances  used  in  his  culture  fluids  for 
Aspergillus  niger,  only  glycerin  was  absorbed  in  sufficient 
quantity  to  be  worthy  of  consideration  as  an  osmotically 
active  solute  within  the  cells. 

c)  Absorption  test. —  If  the  concentration  of  the  sur- 
rounding medium  is  carefully  determined  and  the  organisms 
whose  permeability  is  to  be  tested  be  allowed  to  grow  in  it 
for  a  time,  decrease  in  concentration  of  the  medium  may  be 
interpreted  to  mean  that  absorption  has  taken  place.  This 
has  been  shown  to  occur  in  the  case  of  a  number  of  inorganic 
salts,  but  is  especially  well  marked  with  solutions  of  glucose 
and  glycerin.  Demoussy2  determined  in  this  way  the  relative 
rate  of  absorption  of  potassium  and  calcium  ions  by  wheat, 
maize,  etc.,  while  Laurent3  was  able  to  prove  the  somewhat 
unexpected  fact,  that  roots  of  maize  can  absorb  measurable 
quantities  of  glucose  from  a  solution  in  which  they  are 
grown.  This  is  probably  not  a  general  phenomenon,  for  if 
the  protoplasm  is  permeable  to  sugar  in  one  direction  it  is 
difficult  to  see  how  it  could  fail  to  be  permeable  in  the  oppo- 
site one  also,  and  such  a  condition  must  allow  outward 
diffusion  of  glucoses  and  other  sugars  which  are  found  so 
commonly  in  plant  cells.4 

i  O.  H.  Von  Mayenburg,  "  Losungsconcentration  und  Turgorregulation  bei  den 
Schimmelpilzen,"  Jahrb.f.  wiss.  Bot.,  Vol.  XXXVI  (1901),  pp.  381-420. 

2 E.  Demoussy,  "Absorption  elective  de   quelques  elements  mineraux  paries 
plantes,"  Compt.  rend.,  Vol.  CXXVII  (1900),  pp.  970-73. 

3  J.  Laurent,  "  Sur  l'absorption  des  matieres  organiques  par  les  racines,"  ibid., 
Vol.  CXXV  (1897),  pp.  887-9. 

*  Cf.  Pfeffer-Ewart,  Physiology  of  Plants,  Cambridge,  1900,  p.  99. 


68  Diffusion  and  Osmotic  Peessuke 


That  ordinary  leaves  can  absorb  inorganic  salts  was  shown 
in  this  way  by  Dandeno.1  He  found  that  drops  of  solution 
placed  upon  foliage  leaves  were  completely  absorbed  if  too 
rapid  evaporation  was  prevented.  When  the  drop  disap- 
peared no  trace  of  solute  crystals  remained  upon  the  leaf 

surface. 

d)  Test  by  toxicity. —  To  all  protoplasmic  poisons  must  be 
accredited  power  of  penetration  in  a  greater  or  less  degree ; 
if  there  were  no  penetration  the  substance  could  not  bring 
about  its  toxic  effect.  True2  proved  a  slight  toxicity  for 
KN03  and  NaCl  upon  Spirogyra;  these  substances  must 
therefore  penetrate  the  protoplasts  of  this  plant,  though 
probably  this  occurs  with  difficulty.  More  recently  Coupin3 
has  prepared  a  catalogue  of  the  poisonous  effects  upon  wheat 
of  certain  salts  in  various  concentrations.  His  tables  are 
useful  for  comparison.  Of  course,  the  fact  that  a  rather 
high  concentration  of  a  given  solute  is  needed  to  affect  the 
plant  may  mean  either  that  the  protoplasm  is  only  slightly 
permeable,  or  that  the  substance  is  only  slightly  toxic. 
From  Pfeffer4  we  have  the  fact  that  mercuric  chlorid  and 
iodin  penetrate  many  vegetable  cells  and  exert  a  marked 
toxic  effect. 

There  are  many  other  proofs  that  various  mineral  and 
organic  substances  are  able  to  penetrate  the  plant  protoplast. 
Where  a  noticeable  and  specific  effect  is  produced  upon  the 
organism  by  the  presence  of  a  given  substance  in  the 
medium,  there  can   be   no  doubt  that  the  substance  pene- 

ij.  B.  Dandeno,  "An  Investigation  into  the  Effects  of  Water  and  Aqueous 
Solutions  of  Some  of  the  Common  Inorganic  Substances  on  Foliage  Leaves,"  Tram. 
Canad.  Inst.,  Vol.  VII  (1901),  pp.  238-350. 

2R.  H.  True,  "The  Physiological  Action  of  Certain  Pla»molyzing  Agents," 
Bot.  Gaz.,  Vol.  XXXVI  (1898),  pp.  407-16. 

3H.  Coupin,  "Sur  la  toxicit6  des  composes  du  sodium,  du  potassium,  et  de 
V ammonium  k  l'egard  des  veg6taux  superieurs,"  Rev.  gen.  bot,  Vol.  XII  (1901), 
pp.  177-94. 

*  W.  Pfeffer,  Osmotische  Untersuchungen,  Leipzig,  1877,  p.  140. 


TURGIDITY  69 


trates,  to  some  extent  at  least.  Whether  the  effect  be  the 
death  of  the  plant  or  only  an  alteration  in  its  metabolic 
processes  makes  no  difference  for  the  present  consideration.1 

e)  Test  by  accumulation. — The  power  of  penetration  of  all 
inorganic  salts,  and  of  many  organic  compounds  also,  may 
be  tested  by  analysis  of  plant  material  which  has  been 
grown  in  the  solution  to  be  tested.  The  various  metallic 
ions  such  as  K,  Na,  Ca,  etc.,  are  known  to  accumulate  in  the 
bodies  of  higher  plants.  In  this  manner  Bourget2  demon- 
strated a  marked  absorption  of  iodin  by  various  plant  roots. 
There  is  a  great  difference  in  different  plants  in  this  regard, 
however;  the  Liliaceae  and  Chenopodiaceae  absorb  com- 
paratively large  quantities  of  iodin,  while  Solanum  tuberosum, 
grown  in  the  same  soil,  fails  to  absorb  enough  for  a  test. 

Great  accumulation  of  copper  in  plant  cells  has  been 
recorded  several  times.  Thus,  MacDougal 3  describes  a  case 
where  a  tree  of  Quercus  macrocarpa  absorbed  copper  in 
large  amounts  and  caused  its  precipitation  within  the  wood 
in  the  metallic  state. 

All  these  accumulations  come  about  by  a  chemical  change 
taking  place  in  the  substance  after  it  has  entered  the  cell, 

1  The  following  references  will  be  of  use  to  supplement  those  already  given :  F. 
de  F.  Heald,  "  On  the  Toxic  Effect  of  Dilute  Solutions  of  Acids  and  Salts  upon 
Plants,"  Bot.  Gaz.,  Vol.  XXII  (1896),  pp.  125-53;  H.  M.  Richards,  "Die  Beeinflussung 
des  Wachsthums  einiger  Pilze  durch  chemische  Reize,"  Jahrb.f.  tviss.  Bot..  Vol.  XXX 
(1897),  pp.  665-79;  F.  L.  Stevens,  "The  Effect  of  Aqueous  Solutions  upon  the  (ter- 
mination of  Fungus  Spores,"  Bot.  Gaz. ,Yol.  XXVI  (1898),  pp. 377-406;  E.  B.  Gothland 
and  L.  Kahlenberg,  "  The  Influence  of  the  Presence  of  Pure  Metals  upon  Plants," 
Trans.  Wisconsin  Acad.  Sci.  Arts  and  Let.,  Vol.  XII  (1899),  pp.  454-74;  N.  Ono, 
"  Ueber  die  Wachsthumsbeschleunigung  einiger  Algen  und  Pilze  durch  chemische 
Reize,"  Jour.  Coll.  Sci.  Imp.  Univ.  Tdkrjo,  Vol.  XIII  (1900),  reviewed  in  Bot.  Gaz., 
Vol.  XXX  (1900),  p.  422;  H.  De  Vaux,  "  De  l'absorption  des  poisons  metalliques  tres 
dilues  par  les  cellules  vegetales,"  Compt.  rend.,  Vol.  CXXXII  (1901),  pp.  717-20. 

2  P.  Bourget,  "  Sur  l'absorption  de  Tiode  par  les  vegeteaux,"  Compt.  rend.,  Vol. 
CXXIX  (1899),  pp.  768-70;  idem,  same  title,  Bull,  soc.  chim.  Paris,  Ser.  3,  Vol.  XXIII 
(1899),  pp.  40-41. 

3D.  T.  MacDougal,  "  Copper  in  Plants," Bot.  Gaz.,  Vol.  XXVII  (1899),  p.  68.  He 
cites  the  following  on  the  same  general  subject :  Lehman,  "Der  Kupfergehalt  von 
Pflanzen  und  Thieren  in  kupferreichen  Gegenden,"  Arch,  f.  Hygieru,  Vol.  XXVI 1 
(1896),  p.  1;    J.  B.  Skertschlt,  "Tin  Mines  of  Watsonville,1'    Report    Geologist 

Queensland,  1897. 


70  Diffusion  and  Osmotic  Pressure 

either  within  the  protoplasm  or  in  the  vacuole.  If  this  were 
not  so,  the  diffusion  tension  of  the  solute  would  soon  become 
as  great  within  the  cell  as  without,  and  thus  there  could  be 
no  accumulation.  But  if  a  substance  is  precipitated,  poly- 
merized, or  condensed  within  the  cell  through  the  chemical 
action  of  some  other  body  already  there,  which  perhaps  arises 
as  a  secretion  from  the  protoplasm,  then  the  internal  diffusion 
tension  of  the  entering  substance  will  be  kept  low,  and  inward 
diffusion  will  continue  indefinitely.  In  this  way  copper  salts 
entering  the  cell  are  probably  reduced  to  metallic  copper. 
This  fact  of  accumulation  is  a  very  important  one  in  under- 
standing the  process  of  absorption  of  dissolved  substances  by 
the  plant. 

f)  Test  by  metabolic  processes. — The  absorption  of  any 
food  substance  is  of  course  a  proof  of  permeability  to  that 
substance.  The  immediate  effect  upon  the  living  green  cell 
of  absence  of  carbon  dioxid,  or  upon  any  living  cell  of 
oxygen,  shows  that  these  gases,  when  in  solution,  enter  the 
protoplast  with  extreme  ease.  Penetration  by  many  usually 
solid  substances  may  be  proved  in  this  manner  ;  the  long 
series  of  experiments  upon  growth,  and  especially  the  forma- 
tion of  starch  by  green  plants  in  darkness,  may  be  regarded 
as  evidence  in  this  matter.  Thus,  Bouilhac l  grew  Nostoc  in 
the  dark  in  a  solution  of  glucose,  where  it  appeared  perfectly 
healthy,  and  Artari2  and  Matruchot  and  Molliard3  grew 
Stichococcus  in  organic  solutions  in  a  similar  way.  The 
absorption  of  organic  food  by  many  algse,  and  by  all  saporo- 
phytes  and  parasites,  including  all  of  the  fungi,  may  be 
mentioned  in  this  connection.     In  many  of  these  cases  the 

1 R.  Bouilhac,  "Sur  la  culture  de  Nostoc  punctiforme  en  presence  de  glucose," 
Compt.  rend.,  Vol.  CXXV  (1897),  p.  880. 

2  A.  Abtabi,  "Zur  Ernahrungsphysiologie  der  grunen  Algen,"  Ber.  d.  deutsch. 
hot.  Ges.,  Vol.  XIX  (1901),  pp.  7-10. 

3  L.  Mateuchot  et  M.  Molliaed,  "  Variations  de  structure  d'une  algue  verte 
sous  T influence  du  milieu  nutritif,"  Rev.  gen.  bot.,  Vol.  XL  (1902),  pp.  114-30,  254-68, 
316-32. 


TURGIDITY  71 


presence  of  the  substance  in  question  is  due  to  digestion 
outside  the  body,  brought  about  by  outward  diffusion  of 
enzymes. 

Dandeno1  has  recently  shown  that  inorganic  salts  are 
absorbed  in  some  instances  by  ordinary  leaves  when  these 
are  kept  covered  by  a  solution  by  means  of  a  constant  spray, 
or  by  submersion.  This  occurred  to  such  a  degree  in  this 
writer's  experiments  with  Thunbergia  that  plants  of  this 
form  whose  roots  were  supplied  with  nothing  but  water,  but 
whose  leaves  were  sprayed  with  a  solution,  were  able  to 
make  a  good  growth.  Control  plants,  which  had  distilled 
water  applied  to  the  leaves  as  well  as  to  the  roots,  perished 
in  a  much  shorter  time.  Also,  drops  of  solution  placed  upon 
various  leaves  were  completely  absorbed  if  too  rapid  evapora- 
tion was  prevented.  This  observation  has  been  mentioned 
under  c). 

g)  Outward  permeability . — Many  substances  which  pene- 
trate the  cell  from  without  have  been  shown  to  pass  in  the 
opposite  direction  with  equal  ease.  This  has  been  especially 
emphasized  by  Overton 2  in  the  case  of  the  soluble  alcohols, 
etc.  But  in  general  the  outward  passage  from  the  plant 
body  of  sugars  and  the  various  organic  food  substances  has 
not  been  demonstrated.  It  must  be  of  rather  rare  occurrence, 
or  the  phenomena  of  nutrition,  etc.,  would  be  impossible. 
There  are,  however,  certain  cases  where  exudation  occurs, 
notably  in  the  case  of  glandular  structures,  both  in  plants 
and  animals.  Laurent,3  however,  has  demonstrated  an  out- 
ward passage  of  enzymes  (e.  g.,  amylase  and  sucrase)  from 

i  J.  B.  Dandeno,  "An  Investigation  into  the  Effects  of  Water  and  Aqueous 
Solutions  of  Some  of  the  Common  Inorganic  Substances  on  Foliage  Leaves,"  Trans. 
Canad.  Inst,  Vol.  VII  (1901),  pp.  238-350. 

2  E.  Overton,  "  Ueber  die  osmotischen  Eigenschaften  der  lebenden  PHanzen 
und  Thierzelle,"  Vierteljahrschr.  der  Naturf.-Ges.  in  Zurich,  Vol.  XL  (1895),  pp. 
159-84. 

3  J.  Laurent,  "  Sur  Texosmose  de  diastases  par  les  plantules,"  Compt.  rend., 
Vol.  CXXXI  (1900),  pp.  818-51. 


72  Diffusion  and  Osmotic  Pressure 


the  roots  of  maize  seedlings,  and  a  similar  phenomenon  is 
very  commonly  met  with  in  the  case  of  bacteria,  yeast  fungi, 
etc.  Molisch J  believed  this  to  be  generally  trne,  but  Czapek 2 
has  shown  that  he  was  probably  mistaken.  The  latter  found 
that  normal  roots  give  off  not  only  C02  but  also  phosphoric 
acid  in  the  form  of  an  acid  salt.  These  substances  must  of 
course  pass  out  through  the  protoplasm.  There  seems  to  be 
no  doubt  from  the  work  of  Dandeno3  that  both  organic  and 
inorganic  substances  will  diffuse  out  from  the  cells  of  foliage 
leaves  if  these  are  kept  covered  with  water.  In  these  cases 
inward  and  outward  diffusion  seem  to  take  place  in  exactly 
the  same  manner. 

It  is  a  well-known  fact  that  enzymes,  especially  diastase, 
pass  out  from  the  cells  of  embryos  and  digest  food  stored  in 
the  endosperm  of  seeds.4  This  argues  the  permeability  of 
the  protoplasm  of  both  embryo  and  endosperm  to  these 
substances.  In  this  connection  evidence  has  also  been 
presented  that  the  cells  of  embryo  and  endosperm  are  both 
permeable  to  carbohydrates,  probably  of  the  glucose  group. 
Whether  these  ari^e  from  the  action  of  an  enzyme  derived 
from  the  embryo,  or  from  the  action  of  enzymes  formed 
within  the  endosperm  itself,0  is  of  no  consequence  so  far  as 
permeability  is  concerned;  after  the  carbohydrates  are 
formed  they  diffuse  into  the  embryo. 

h)   Variations  in  permeability. — If  a  turgid  cell  gives 

1H.  Molisch,  "Ueber  Wurzelausscheidungen  und  deren  Einwirkung  auf  orga- 
nische  Substanzen,"  Sitzungsber.  d.  kais.  Akad.  d.  Wiss.  zu  Wien,  math.-nat.  hist. 
Klasse,  Vol.  XCVI  (1887),  pp.  84-109. 

2F.  Czapek,  "Zur  Lehre  von  den  Wurzelausscheidungen,"  Jahrb.f.  wiss.  Bot., 
Vol.  XXIX,  pp.  321-90. 

3Loc.  cit.,p.  71. 

4Gkuss,  "  Ueber  d.  Eintritt  von  Diastase  in  d.  Endosperm.,"  Ber.  d.  deutsch.  bot. 
Ges.  zu  Berlin,  Vol.  IV  (1893),  p.  286;  also  B.  Hansteen,  "  Ueber  die  Ursachen 
der  Entleerung  der  Reservestoffe  aus  Samen,"  Flora,  Vol.  LXXIX  (1894),  pp.  419-29. 

5  K.  Pukiewitch,  "Ueber  die  selbstthatige  Entleerung  der  Reservestoffbehalter," 
Ber.  d.  deutsch.  bot.  Ges.,  Vol.  XIV  (1896),  pp.  207-15;  idem,  "  Physiologische  Unter- 
suchungen  uber  die  Entleerung  der  Reservestoffbehalter,"  Jahrb.  f.  wiss.  Bot.,  Vol. 
XXXI  (1897),  pp.  1-76. 


TURGIDITT  73 


out  pure  water,  this  occurrence  must  be  due  to  one  of  two 
conditions:  (1)  the  protoplasm  may  contract  with  great  force, 
thus  overcoming  the  osmotic  pressure  of  the  contained 
solutes  and  causing  the  solvent  to  pass  outward  through 
the  membrane,  or  (2)  a  relative  decrease  in  the  internal 
pressure  may  occur  resulting  either  from  an  active  precipita- 
tion or  condensation  of  some  of  the  solutes  of  the  sap,  or  from 
an  absolute  rise  in  the  external  osmotic  pressure.  Accord- 
ing to  supposition  (1),  the  osmotic  pressure  within  the  cell 
remains  unchanged,  but  is  in  part  overcome  by  the  mechani- 
cal pressure  of  the  contracting  protoplasmic  membrane. 
According  to  supposition  (2),  the  internal  osmotic  pressure  is 
relatively  reduced,  and  the  protoplasm  does  not  exert  any 
appreciable  pressure  itself,  but  is  forced  inward  through  the 
solvent  by  the  osmotic  pressure  of  the  solutes  outside  the  cell 
and  by  the  elastic  force  of  the  restraining  cellulose  wall. 
It  is  probable  that  this  last  supposition  expresses  the  truth  in 
many  cases  where  an  alteration  in  turgidity  is  observed. 
The  former  supposition  is  not  tenable  at  all ;  the  protoplast 
would  burst  long  before  concentration  of  the  sap  solution 
could  be  brought  about  by  pressure. 

If  a  cell  gives  out  a  solution,  the  cause  of  this  must  be  a 
change  in  the  permeability  of  the  protoplasm,  such  that  it 
now  allows  the  outward  passage  of  solutes  to  which  it  was 
formerly  impermeable.  The  liquid  exuded  in  guttation  is 
known1  to  be,  not  pure  water,  but  a  portion  of  the  cell  sap. 
In  the  last  particular  this  sort  of  shrinkage  of  the  vacuole 
differs  from  true  plasmolysis,  for  in  that  we  have  the  extrac- 
tion of  pure  water.  However,  the  apparent  effect  upon  the 
cell  is  the  same ;  if  the  volume  of  the  vacuole  is  in  any  way 
decreased,  the  protoplasmic  sac  will  contract  from  its  own 
elasticity  and  surface  tension,  if  for  no  other  reason. 

*G.  Bonniee,  "  Recherches  experimentales  sur  la  miellee,"  Rev.  gen.  bot.,  Vol. 
VIII  (1896),  pp.  1-22;  Dandeno  has  shown  that  guttation  droplets  contain  both 
organic  and  inorganic  solutes.    See  Trans.  Canacl.  Inst.,  Vol.  VII  (1901),  pp.  238-3J0. 


74  Diffusion  and  Osmotic  Pressure 

At  different  times  and  under  different  conditions  the 
permeability  of  certain  protoplasts  apparently  changes 
greatly.  The  presence  of  poisons  may  cause  the  protoplasm 
to  become  more  permeable  to  other  substances.  Thus, 
Maquenne1  found  that  HgCl2  caused  a  marked  increase  in 
the  permeability  of  the  protoplasm  of  the  cells  of  Helianthus 
seedlings  to  plasmolyzing  agents.  Similarly  DeVries2  found 
that  plasmatic  membranes  which  were  normally  impermeable 
to  KN03  and  NaCl  could  often  be  made  permeable  to  them 
by  treatment  with  an  acid  or  a  base.  With  animal  muscle 
Loeb3  has  shown  that  acids,  bases,  and  other  chemicals  exert 
a  great  influence  upon  the  water-absorbing  power  of  the 
cells.     This  may  be  due  to  changes  in  permeability. 

Partial  or  complete  plasmolysis  may  act  in  the  same  way. 
Oltmanns  *  was  able  to  cause  Fucus  cells  to  give  out  coloring 
matter  by  placing  the  tissues  in  concentrated  solutions. 
Both  of  these  reactions  must  consist  in  an  alteration  of  the 
physical  (perhaps  chemical)  structure  of  the  protoplasm. 
On  the  other  hand,  an  increase  in  turgor  above  the  normal 
may  cause  the  same  change.  This  seems  to  be  the  case  in 
the  guttation  from  the  water  pores  of  the  leaves  of  the 
tomato,  balsam,  etc.  When  the  turgor  pressure  in  the 
cells  bordering  these  water  pores  passes  a  certain  limit,  the 
protoplasm  apparently  becomes  altered  so  that  the  cell  sap 
oozes  out  and  appears  in  droplets  on  the  leaf-tips  where 
water  pores  are  present.     Czapek5  describes  a  similar  phe- 

1L.  Maquenne,  "Sur  la  pression  osmotique  dans  les  graines  germees,"  Com.pt. 
rend.,  Vol.  CXXIII  (1896),  pp.  898,  899. 

2  H.  De  Veies,  "  Plasmolytische  Studien  fi.  d.  Wand  d.  Vacuolen,"  Jahrb.  f.  wiss. 
Bot.,  Vol.  XVI  (1885),  pp.  465-598. 

3  J.  Loeb,  "  Physiologische  Untersuchungen  fiber  Ionenwirkung":  I.  Mitthei- 
lung,  "  Versuche  am  Muskel,11  Pflugers  Arch.f.  d.  ges.  Physiol.,  Vol.  LXIX  (1897),  pp. 
1-27. 

4  F.  Oltmanns,  "  Ueber  die  Bedeutung  der  Concentrationsanderung  des 
Meerwassers  ffir  das  Leben  der  Algen,"  Sitzungsber.  d.  k.  preuss.  Akad.  d.  Wiss.  zu 
Berlin,  Vol.  X  (1891),  p.  183. 

5F.  Czapek,  "Zur  Lehre  von  den  Wurzelausscheidungen,"  Jahrb.  f.  wiss.  Bot., 
Vol.  XXIX  (1896),  pp.  321-90. 


TURGIDITY  75 


nomenon  in  the  case  of  turgid  root  hairs,  from  which  droplets 
of  solution  are  exuded.  It  is  probable  that  in  these  cases 
we  have  to  do  with  a  phenomenon  related  to  glandular 
secretion. 

Another  potent  cause  for  great  increase  in  protoplasmic 
permeability  in  some  instances  is  lowering  of  temperature. 
If  a  filament  of  any  common  alga  be  carefully  dried  exter- 
nally and  placed  in  olive  oil  whose  temperature  is  then 
rapidly  lowered  to  the  vicinity  of  0°  C,  a  film  of  water  may 
be  seen  to  form  about  the  filament,  and  partial  plasmolysis 
may  be  observed.  When  the  temperature  is  again  brought 
back  to  normal,  the  extruded  water  is  again  absorbed. 
Greeley 1  has  recently  shown,  not  only  that  complete  plas- 
molysis can  be  produced  in  Spirogyra  by  low  temperature, 
but  that  the  same  thing  occurs  in  Stentor  coeruleus.  Exactly 
the  same  phenomenon  is  exhibited  by  Stentor  individuals 
when  water  is  removed  from  them  by  the  action  of  a  con- 
centrated sugar  solution.  The  animals  plasmolyzed  by  low 
temperature  return  to  their  normal  activity  with  rise  in  tem- 
perature, but  Greeley  was  unable  to  cause  the  same  reversal 
in  the  case  of  the  osmotically  plasmolyzed  individuals.  I 
have  often  observed  that  the  liquid  exuded  from  cells  of 
Spirogyra  plasmolyzed  by  cold  is  a  solution.  Its  freezing 
point  is  considerably  lower  than  that  of  pure  water. 

The  theory  of  death  by  freezing  which  was  advanced  by 
Molisch2  accounts  for  the  decline  of  activity  and  for  final 
death  at  low  temperatures  by  the  extraction  of  water  from 
the  protoplasm  until  the  processes  which  make  up  life  are 
no  longer  possible.     Matruchot  and  Molliard3  have  pointed 

i  A.  W.  Greeley,  "  On  the  Analogy  between  the  Effects  of  Loss  of  Water  and 
Lowering  of  Temperature,"  Am.  Jour.  Physiol.,  Vol.  VI   (1901),  pp.  112-28. 

2H.  Molisch,  Untersuchungen  iiber  das  Erfriercnder  Pflanzcn,  Jena,  1897  { 
reviewed  in  Bot.  Centralbl,  Vol.  LXXIII  (1898),  p.  149. 

3L.  Matrochot  and  M.  Molliard,  "Sur  Tidentite  des  modifications  de 
structure  produites  dans  les  cellules  vegetales  par  le  gel,  la  plasmolyse,  et  la 
fanaison,"  Compt.  rend.,  Vol.  CXXXII  (1901),  pp.  495-8. 


76  Diffusion  and  Osmotic  Pressure 

out  a  striking  parallelism  in  the  behavior  of  plant  nuclei 
which  have  been  either  frozen,  dried,  or  subjected  to  the 
osmotic  action  of  a  concentrated  solution.  In  all  these  cases 
water  was  found  to  be  extruded  from  the  nucleus.  The 
nuclear  material  took  on  a  peculiar  appearance  not  unlike 
that  of  karyokinetic  figures. 

Krabbe 1  experimented  upon  the  effect  of  rise  in  tempera- 
ture upon  the  absorption  of  water  by  various  plant  cells, 
finding  that  the  rate  of  absorption  rises  with  the  tempera- 
ture. This  author  supposes  the  response  to  be  due  to  a 
physical  change  in  the  protoplasm,  caused  by  the  higher 
temperature. 

But  the  best  series  of  experiments  on  the  change  in  pro- 
toplasmic permeability  due  to  temperature  variations  is  that 
of  van  Rysselberghe.2  He  worked  with  a  variety  of  plant 
cells  (Sambucus,  Tradescantia,  Begonia,  Lemna,  green  algse, 
etc.)  and  found  that  Krabbe' s  general  result  is  true.  The  ratio 
of  increase  in  permeability  to  water  becomes  less,  however,  as 
the  temperature  rises.  From  0°  C.  to  5°  C,  this  ratio  is  0.05 ; 
from  5°  Cto  18°  C,  0.043 ;  and  above  the  last-named  tempera- 
ture, 0.1.  The  ratios  between  the  permeability  to  water  at 
0°  and  that  at  6°,  12°,  16°,  20°,  25°,  30° 

are:    1, 2,    4.5,      6,       7,  7.5,      8. 

The  total  amount  of  water  absorbed  by  a  cell  is  not  changed 
by  variations  in  temperature;  the  rate  of  absorption  alone  is 
affected.  The  nature  of  the  protoplasm  does  not  appear  to 
have  any  effect  on  the  total  amount  of  water  absorbed  or 
given  out  by  a  cell ;  this  is  determined  by  the  osmotic  pres- 
sure of  the  sap  and  by  the  temperature.     Permeability  to 

1G.  Krabbe,  "Ueber  d.  Einfluss  d.  Temperatur  auf  d.  osmotische  Processe 
lebender  Zellen,"  Jahrb.f.  tviss.  Bot.,  Vol.  XXIX  (1896),  pp.  441-98. 

2F.  van  Rysselberghe,  "Influence  de  la  temperature  sur  la  permeability  du 
protoplasme  vivant  pour  l'eau  et  les  substances  dissoutes,"  Recueil  de  Vinst.  hot.  de 
Bruxelles,  Vol.  V  (1901),  pp.  209-49;  idem,  "  Reaction  osmotique  des  cellules  vegetales 
a  la  concentration  du  milieu,"  Mem.  cour.  pub.  par  Vacad.  roy.  de  Belg.,  Vol.  LVIII 
(1898),  pp.  1-101. 


TURGIDITY  77 


this  liquid  does  not  cease  altogether  at  0°  C,  as  was  thought 
by  Schwendener.1  At  0°  C.  protoplasm  is  not  only  perme- 
able to  water,  but  also  to  KN03,  glycerin,  urea,  methylene 
blue,  caffem,  and  ammonium  carbonate.  Thus  Krabbe's 
idea  that  below  5°  C.  nothing  but  water  penetrates  is  entirely 
unfounded.  Temperature  variations  in  permeability  to 
solutes  were  also  observed  in  many  cells  by  the  same  author. 
These,  too,  seem  to  follow  the  rule  for  water  as  stated  on  the 
preceding  page. 

However,  Copeland2  showed  that  decrease  in  temperature 
caused  a  rise  in  tde  turgor  pressure  of  moss  leaves.  This 
may  not  be  a  direct  effect  of  the  temperature  upon  the  pro- 
toplast, for  the  same  author  found  that  various  agencies 
which  checked  growth  also  caused  a  rise  in  turgor ;  there  is 
surely  a  close  relation  between  growth  and  turgor,  what- 
ever this  relation  may  ultimately  turn  out  to  be.  In  this 
connection  it  may  be  noted  that  De  Vries3  found  that,  as 
growth  proceeds,  turgor  rises,  to  fall  again  after  the  curve 
of  growth  begins  to  decline. 

The  extrusion  of  liquid  from  the  cells  of  the  pulvini  of 
"sensitive"  organs,  such  as  the  leaves  of  Mimosa  and  the 
stamens  of  Berberis,  may  be  due  to  a  change  in  permeability 
also.  There  seems  to  be  a  question,  however,  as  to  whether 
the  exuded  liquid  is  water  or  a  solution.  Pfeffer4  considers 
this  subject  at  some  length,  and  concludes  that  solutes  prob- 
ably do  not  pass  out.  He  believes  that  the  salts  found  by 
Janse5  in  the  extruded  liquid  from  pulvini  in  Mimosa  are 

IS.  Schwendener,  " Zur  Kritik der  neuesten Untersuchungen  a.  d.  Saftsteigen  " 
bitzungsber.  d.  k.  preuss.  Akad.  d.  Wiss.  zu  Berlin,  Vol.  VI  (1892),  p.  911. 

2E.  B.  Copeland,  Ueber  den  Emfluss  von  Licht  u.  Tempiratur  auf  den  Turgor, 
-H.ct.LLe  a.  o.,  1896. 

3  H    De  Vries,  "  Ueber  die  Ausdehnung  wachsender  Pflauzenzellen  durch  ihren 
Turgor,"  Bot.  Zeitg.,  Vol.  XXXV  (1877),  p.  1. 

*  W.  Pfeffer,  "Zur  Kenntniss  der  Plasmahaut  u.  d.  Vacuolen,  etc.,"  Abhandl 
d.  k.  sacks.  Ges.  d.  Wiss.,  math.-physik.  Klasse,  Vol.  CLXI  (1890),  pp.  187-344 

5  J.  M.  Janse,  "  Die  Permeabilitat  des  Protoplasma,"  Verslag.  en  Mclolccl.  d. 
k.  Akad.  v.  Wetensch.  te  Amsterdam,  1888. 


78  Diffusion  and  Osmotic  Pressure 


merely  extra-cellular  material.  But  Pfeffer  shows  that  there 
is  surely  an  extrusion  of  solutes  from  the  stamens  of  Cynara. 
It  may  be  that  all  "sensitive"  organs  do  not  act  alike  in 
this  regard.  Hilburg1  states  that  the  cells  of  the  pulvini  of 
leaves  of  Phaseolus  increase  in  turgor  pressure  when  sub- 
jected to  the  action  of  light. 

Puriewitch2  found  that  absence  of  oxygen  and  the  pres- 
ence of  anesthetics  prevented  the  giving  off  of  reserve  food 
from  cells  of  tubers,  roots,  bulbs,  the  endosperm  of  seeds,  etc. 
He  explains  this  fact  as  an  effect  upon  the  diastatic  enzymes 
in  the  various  cases,  but  the  onion  bulb  and  the  beet  root 
exhibit  the  same  phenomenon,  and  it  is  difficult  to  see  how 
organs  whose  stored  food  is  already  in  solution  could  be 
affected  by  an  alteration  in  enzymes— unless,  indeed,  even 
sugars  need  to  be  modified  by  enzyme  action  before  they 
can  pass  the  protoplasm.  A  live  onion  scale  totally  submerged 
in  water  will  give  off  no  sugar,  but  if  partially  exposed  to 
the  air  exudation  takes  place.  It  is  probable  that  here  we 
have  another  case  of  the  influence  of  a  chemical  (i.  e., 
oxygen)  upon  the  permeability  of  the  membrane.  That 
enzyme  action  is  necessary  for  the  translocation  of  the  cane 
sugar  of  the  beet  root  is  possible  and  even  probable. 

The  permeability  of  the  protoplast  to  a  certain  substance 
may  change  according  to  the  relative  concentration  of  that 
substance  within  and  without  the  cell,  as  has  been  shown 
recently  by  Nathansohn.3  Codium  tomentosum  wtis  used  in 
his  experiments.  The  cells  of  this  plant  are  permeable  to 
chlorids,  so  that  the  content  of  HC1  is  found  to  be  the  same 
in  expressed  sap  and  in   the  surrounding  medium.     If  the 

i  C.  Hilburg,  "  Ueber  Turgescenczanderungen  in  den  Zellen  der    Bewegungs- 
gelenke,"  Unters.  aus  d.  bot.  Inst,  zu  Tubingen,  Vol.  I  (1881),  pp.  23-52. 

2  K.  Puriewitch,  "  Physiologische  Untersuchungen  u.  d.  Entleerung  der  Reser- 
vestoffbehalter,"  Jahrb.  f.  wiss.  Bot.,  Vol.  XXXI  (1897),  pp.  1-76. 

3  A.  Nathansohn,  "Zur  Lehre  von  Stoffaustausch,"  Ber.  d.  deutsch.  bot.  Ges., 
Vol.  XIX  (1901),  pp.  509-12. 


TURGIDITY  79 


plant  be  put  into  a  solution  without  the  chlorid,  this  sub- 
stance diffuses  out  of  the  cells  in  twelve  to  twenty-four 
hours,  and  the  reverse  occurs  if  the  external  solution  con- 
tains more  CI  ions  than  the  internal.  Other  substances  in 
the  medium  will  cause  the  retention  of  chlorid,  apparently 
in  inverse  proportion  to  their  ability  to  penetrate.  The 
following  table  to  show  this  is  taken  from  Nathansohn's 
paper: 

Medium  Pressure    HC1  Content 

per  cent. 

Normal  Sea  H20  Sw1  2.25 

Na  N03  iSw              .67 

Na  N03  Sw  1.27 

Urea  Sw  0.64 

Glycerin  Sw  1.0 

Grape  sugar  |Sw  1.45 

Of  the  substances  named  in  the  table,  those  of  least  pene- 
trating power  are  grape  sugar  and  NaN03.  This  is  a  new 
departure,  and  no  definite  conclusions  can  be  drawn  until 
more  work  has  been  done. 

Very  clear  evidence  of  change  in  permeability  of  proto- 
plasm to  solutes  is  that  obtained  by  Haupt2  in  his  work  on 
extrafloral  nectaries.  In  certain  plants  (e.  g.,  Euphorbia 
and  Vicia)  he  found  that  the  secretion  of  sugar  by  the  nec- 
taries was  profoundly  influenced  by  light,  specifically  by  the 
red-yellow  rays  of  the  sun's  spectrum.  Secretion  of  sugar 
occurred  only  in  light,  and  from  nectaries  already  containing 
sugar  this  substance  was  resorbed  in  darkness  or  in  blue 
light.  This  must  indicate  that  the  nature  of  the  protoplas- 
mic membranes  here  is  entirely  altered  by  the  etheric  vibra- 
tions; in  light  sugar  passes  outward,  while  in  darkness  it 
moves  in  the  opposite   direction.     That   plants   which   had 

1  Sw  in  the  table  denotes  the  osmotic  pressure  of  normal  sea-water. 

2H.  Haupt,  "  Zur  Secretionsmechanik  der  extrafloralen  Nektarien,"  Flora,  Vol. 
X  (1902),  pp.  1-41. 


80  Diffusion  and  Osmotic  Pressure 

been  deprived  of  C02  for  many  days  still  exhibited  this 
response  seems  to  show  that  the  phenomenon  stands  in  no 
direct  relation  to  the  process  of  photosynthesis. 

IV.       ACTION    OF    THE    PROTOPLASMIC    MEMBRANE 

If  ignorance  still  prevails  regarding  the  manner  in  which 
a  purely  physical  osmotic  membrane  acts,  it  is  even  more 
prevalent  regarding  the  action  of  the  protoplasmic  mem- 
brane of  the  living  cell.  That  the  tonoplast  and  ectoplast 
are  the  main  factors  in  the  production  of  semi-permeability 
there  seems  little  reason  to  doubt.  There  are  three  possible 
ways  in  which  they  may  act,  and  of  course  the  same  mem- 
brane may  act  in  different  ways  at  the  same  time.  These 
three  possible  explanations  of  semi-permeability  may  be  briefly 
stated  as  follows: 

a)  The  filter  theory. —  The  simplest  explanation  of 
osmotic  pressure,  whether  within  a  living  cell  or  not,  is  this: 
That  the  semi-permeable  membrane  acts  merely  as  a  sieve 
or  filter,  and  that  the  larger  solute  particles  are  prevented 
from  passing  by  the  smallness  of  the  opening,  while  the 
smaller  solvent  particles  pass  with  only  slightly  increased 
friction.  Since  we  cannot  be  sure  of  the  relative  sizes 
of  the  different  molecules  and  ions,  there  is  no  way  of 
directly  testing  this  hypothesis.  But  there  are  many  facts, 
both  in  chemistry  and  physiology,  which  make  the  filter 
theory  at  least  very  improbable  in  many  instances.  Among 
the  physiological  facts  which  tend  in  this  direction  may  be 
cited  the  effects  of  chemicals  in  varying  the  permeability  of 
the  protoplasm,  especially  the  observations  of  Maquenne, 
De  Vries,  and  Loeb,  already  discussed1  and  the  work  of 
Overton,  to  be  taken  up  under  b). 

b)  The  solution  theory. — According  to  this  theory,  the 
membrane  is  to  be  considered  as  a  solvent  in  which  the  pene- 

1  See  above,  p.  74. 


TURGIDITY  si 


trating  substances  are  readily  soluble,  while  those  which  fail 
to  penetrate  are  not.  Thus,  in  the  case  of  an  osmotic  cell 
containing  a  sugar  solution  and  surrounded  by  pure  water, 
it  may  be  supposed  that  the  membrane  dissolves  water 
readily,  but  cannot  dissolve  sugar.  Thus,  water  would  go 
into  solution  in  the  membrane  on  the  side  of  higher  diffusion 
tension  of  water  (the  side  of  the  solvent),  and,  after  diffusing 
through  the  membrane,  would  be  given  off  on  the  side  of 
lower  diffusion  tension  of  that  substance  (the  side  of  the 
solution).  The  solute,  not  being  able  to  go  into  solution  in 
the  membrane,  merely  exerts  its  expansive  force  upon  it,  and 
does  not  diffuse. 

From  the  researches  of  Overton1  and  others  it  appears 
that  this  is  probably  the  true  explanation  of  many  cases  of 
the  development  of  osmotic  pressure  in  plant  and  animal 
cells.  By  comparing  the  numerous  substances  which  he 
found  able  to  penetrate  the  protoplasm,  Overton  observed 
that  penetrating  power  seemed  to  vary  in  proportion  to  solu- 
bility in  aliphatic  oils  and  ethers.  He  went  still  further:  A 
given  substance,  like  glycerin,  may  be  only  slightly  soluble 
in  aliphatic  oils,  and  may  penetrate  the  protoplast  slowly; 
but  if  its  substitution  products  (as  mono-  and  dichlor-hydrin, 
or  mono-  or  dimethyl-glycerin)  are  more  soluble  in  this 
class  of  substances,  their  power  of  penetrating  the  plant 
protoplast  is  also  found  to  be  greater,  and  this  in  proportion 
as  solubility  has  been  increased  by  the  substitution.  From 
these  and  many  other  observations  Overton  is  led  to  con- 
clude that  the  peculiar  property  of  the  protoplast  of  being 
permeable  to  certain  substances  and  not  to  others  probably 
depends  upon  the  presence  within  it  of  some  aliphatic  oil 
or  ester,  or  a  mixture  of  these  substances.  He  goes  so  far 
as  to  point  out  that  cholesterin  and  the  lecithins,  substances 

1  E.  Overton,  "Ueber  die  allgemeinen  osmotischen  Eigenschaften  der  Zello, 
ihre  vermuthlichen  Ursachen  und  ihre  Bedeutung  fur  die  Physiologie,"  Vierteljahr- 
schr.  d.Naturf.-Ges.  in  Zurich,  Vol.  XLIV  (1899),  pp.  88-135. 


82  Diffusion  and  Osmotic  Pressure 


of  this  nature  which  are  of  general  occurrence  in  animal  and 
plant  cells,  when  in  certain  mixtures,  absorb  water.  He 
suggests  that  the  tonoplast  and  ectoplast  may  be  merely 
layers  of  such  substances,  and  gives  reasons  why  it  is  not 
plausible  to  suppose  that  a  simple  aliphatic  oil  plays  this 
part  of  surface  layer,  as  was  supposed  by  Quincke.1  The 
difference,  however,  between  oils  and  lecithins  is  so  slight 
that  Quincke's  theory  is  not  fundamentally  modified  by 
these  facts. 

On  the  purely  physical  side  Meerburg's  work2  on  the 
passage  of  fuchsin  through  membranes  of  copper  ferro- 
cyanid  seems  to  present  evidence  in  favor  of  the  solution 
theory.  The  dye  could  not  pass  the  membrane,  even  in  a 
slight  degree,  until  the  latter  had  become  fully  impregnated 
with  it.  Flusin  also  showed 3  that  the  rapidity  of  diffusion 
of  various  substances  through  a  membrane  of  caoutchouc 
varies  in  the  same  degree  as  does  the  absorptive  power  of 
the  membrane  for  these  substances.  The  same  was  found 
to  be  true  for  pig's  bladder. 

c)  The  chemical  theory. — The  chemical  explanation  of 
the  phenomenon  of  semi-permeability  supposes  that  the 
membrane  takes  an  active  chemical  part  in  the  transmission 
of  substances  from  one  side  to  the  other.  In  all  its  free- 
dom of  indefiniteness  this  theory  offers  the  only  escape  from 
the  dark  problem  of  glandular  secretion,  wherein  the  solute 
moves  from  the  weaker  to  the  stronger  solution  against  its 
own  diffusion  tension.  This  is  one  of  the  most  difficult  of 
physiological  problems,  and  upon  it  absolutely  no  light  has 

i  Quincke,  "  Ueber  Emulsionsbildung  und  d.  Einfluss  der  Galle  bei  der  Ver- 
dauung,"  Pflilgers  Arch.f.  d.  fires.  Physiol,  Vol.  XIX  (1879),  pp.  129-44. 

2  J.  H.  Meeebueg,  "Zur  Abhandlung  Tammanns :  Ueber  die  Permeabilitat  der 
Niederschlagsmembranen,"  Zeitschr.f.  physik.  Chem.,  Vol.  XI  (1893),  p.  446-8. 

3  G.  Flusin,  "  Sur  Fosmose  des  liquides  &  travers  une  membrane  de  caoutchouc 
vulcanise,"  Compt.  rend.,  Vol.  CXXVI  (1898),  pp.  1497-1500;  idem,  "Sur  Fosmose  des 
liquides  &  travers  une  membrane  de  vessie  de  pore,''  ibid.,  Vol.  CXXXI  (1900),  pp. 
1308, 1309. 


TURGIDITY  go 

yet  been  shed.  In  the  realm  of  puT^cT^n^ 
there  is  some  evidence  that  osmotic  membranes  may  some- 
times play  a  chemical  part.1 

V.       THE  NATURE  OF  THE  OSMOTIOALLY  ACTIVE  SOLUTES 

Within  the  cell  sap  there  are  very  many  substances  in  solu- 
tion, and  thus  the  question  as  to  what  substances  the  turgor 
pressure   is   due   becomes   of   some  importance.     DeVrfes2 
found  that  in  the  onion  bulb-scale  and  in  the  beet  root  this 
pressure  is  chiefly  due  to  stored   sugar.      He  also  showed 
that  the  salts  KC1  and  KN03  play  important  roles  in  the 
maintenance  of  turgor  in  leaf-stalks  of  Gunnera  scabra  and 
the  shoots  of  Helianthus  tuberosus,  respectively.     Copeland3 
concluded  from  experiments  with  Phaseolus,  Pisum   Fa-o- 
pyrum,  and  Zea,  that  in  these  plants  the  osmotically  active  sub- 
stance is  mainly  potassium  nitrate.  On  the  other  hand  Kraus* 
and  DeVries5  found  that- in  many  plants  organic  acids  are 
the  mam  source  of  the  osmotic  pressure  of  the   cell  sap 
I  he  research  of   von   Mayenburg6  shows   that  Aspergillus 

'On  the  action  of  animal  membranes  in  diffusion  phenomena  see  the  Ihlln-i— . 

H.  J.  Hambdegee,  "  Die  isotonischen  Koefflzienten  end  die  rotten  Blutto™  "l     * 
Zettschr.  f.physik.  Chem.,  Vol.  VI  (1890),  nn   319-33.  mrJ    * 7 k    B'utk°rI>"che'>. 
intra-abdominalen  Drucke.  anf  die  Resorp  ion^n  do" :S«lS.He  ""lea ^  "? 

G   PldsiJ     Wl^'  ,  ?"     °  ^t,on  °f  the  copper  fe"«>y«nW  membrane 

c0:zz,  v-oicSii  <i^,TPs  ^t"™"  de  f™e  d° "*» >" 

Vol.  XIT«PP.  mS.  Meth°de  ~  AEal,Z0  d»T"*«k»ft.'V«-r»./.  .ta  M. 

4lBSwB' Eolatioaof  Nutrient  Salts  to  Turgor-"  *«■  «-.** 

4G.  Kraus,  Stoffwechsel  der  Crassulaceen,  1886. 
XXXVH  n^EIES'«^fer  d'  BedTeutu^  d-  PAanzensfturen,"  etc.,  Bot.  Zci,,.  Vol. 

S,M6°'  ? 'T°N  ^A/ENBUEG'  "Losungscoocentration  und  Turgorregulation  bei  den 
Schimmelpilzen,"  JoArfc/.  «,«».  i?0*M  Vol.  XXXVI  (1901),  pp.  3S1-4LU 


84  Diffusion  and  Osmotic  Pkessuee 


niger,  when  grown  in  strong  nutrient  solutions  of  sugar,  etc., 
escapes  plasmolysis  by  an  enormous  increase  in  concentra- 
tion of  the  cell  sap,  this  being  produced  in  part  by  true 
carbohydrates,  but  mainly  by  some  unidentified  substance, 
which  is,  however,  probably  closely  related  to  the  latter 
group.  Maquenne1  found  by  the  freezing-point  method 
that  the  expressed  juice  of  seedling  peas  six  days  old  had  an 
osmotic  pressure  such  that  the  average  molecular  weight  of 
the  solutes  must  be  in  the  neighborhood  of  239.  This  shows 
that  the  active  substance  has  a  much  larger  molecule  than 
glucose  (mol.  wt.  180).  The  juice  failed  to  show  the  pres- 
ence of  glucose  or  cane  sugar  by  tests  made  with  Fehling's 
solution  and  with  phenylhydrazin  acetate.  Helianthus  seed- 
lings, however,  showed  the  presence  of  glucose,  and  the  sap 
had  an  average  molecular  weight  of  only  136.  Here,  then, 
the  pressure  is  largely  produced  by  some  molecule  much 
smaller  than  that  of  glucose.  Stange2  found  that  the  cells 
of  growing  root  tips  of  Pisum,  Lupinus,  etc.,  which  had  been 
grown  in  strong  KN03  solutions,  showed  no  accumulation 
of  that  salt,  although  there  was  a  marked  increase  of  it  in 
the  parenchyma  farther  up.  The  turgor  pressure  was  the 
same  in  the  root  tip  as  elsewhere  in  the  plant.  This  argues 
that  the  increase  in  turgor  in  the  growing  region,  which 
prevents  plasmolysis  where  the  culture  is  in  a  strong  solu- 
tion, must  be  due  to  some  other  substance  or  substances.  A 
collection  of  analyses  of  various  plant  parts,  which  may  be 
taken  as  at  least  some  indication  of  the  nature  of  the  active 
substances  in  the  sap,  is  given  by  Mayer.3     DeVries4  made 

1L.  Maquenne,  "Sur  la  pression  osmotique  dans  les  graines  germees,"  Compt. 
rend.,  Vol.  CXXIII  (1896),  pp.  898  ff. 

2B.    Stange,    "Beziehungen    zwischen    Substratconcentration,     Turgor   und 
Wachsthum  bei  einigen  phanerogamen  Pflanzen,"  Bot.  Zeitg.,  Vol.  L  (1892),  p.  253. 

3  A.  Mayer,  Lehrbuch  der  Agriculturchemie,  Heidelberg,  1895,  pp.  307  ff. 

*H.  DeVeies,  uEine  Methode  zur  Analyse  der  Turgorkraft,"  Ja/u-6./.  wiss.  Bot., 
Vol.  XIV  (1884),  pp.  427-601. 


TURGIDITY  85 


analyses  of  the  sap  itself  and  his  results  show  that  different 
plants  vary  much  as  to  the  nature  of  their  osniotically 
active  substances. 

Heald1  has  recently  determined  the  electrical  conductivity 
of  sap  expressed  from  the  roots,  stems,  and  leaves  of  various 
plants.  The  amount  of  electrolytes  thus  indicated  is  in 
reasonably  close  agreement  with  the  amount  of  ash  found  by 
incineration.  This  only  goes  to  show  that  most  of  the 
electrolyte  molecules  are  dissociated  in  the  sap,  and  are 
therefore  active  in  conducting  the  current.  Any  conclusions 
with  regard  to  the  osmotic  pressure  of  the  sap  which  are 
based  on  conductivity  methods  must  be  absolutely  unreliable, 
unless  it  is  first  ascertained  that  there  are  no  non-conductors 
present,  and  also  that  the  electrolytes  present  are  in  the  ionic 
condition.  But  it  is  probably  impossible  to  find  a  natural 
plant  juice  whose  solutes  are  all  electrolytes.  Therefore 
Heald's  method  cannot  be  of  use  in  determining  osmotic 
pressures.  The  freezing-point,  boiling-point,  and  vapor  ten- 
sion methods  are  applicable  to  this  problem,  however. 

Maquenne's  work  on  the  freezing-points  of  plant  juices2 
has  recently  been  added  to  by  Sutherst.3  The  latter  author 
has  merely  given  the  freezing-points,  without  determining 
the  weight  per  liter,  so  that  his  results  will  not  be  available 
for  any  determination  of  the  nature  of  the  solutes.  In  the 
following  table,  taken  from  his  paper,  I  have  calculated  the 
osmotic  pressures  to  facilitate  comparison: 

TT         ,    ,  t  pv  r»f  Os.  Pressure  in 

Vegetable  marrow:  *r- Ft-  mm.  of  Hg. 

Leaf  and  stalk -0.75°  6,880 

Fruit -0.75°  6,880 

Swede  turnip: 

Entire  plant         -  -  -1.0°  9,173.2 

IF.  de  F.  Heald,  "The  Electrical  Conductivity  of  Plant  Juices,"  Science,  N.  S., 
Vol.  XV  (1902),  p.  457;  idem,  same  title,  Bot.  Gaz.,  Vol.  XXXIV  (1902),  pp.  81-i*2. 

2  See  above,  p.  84. 

3  W.  T.  Sutherst,  "  The  Freezing  Point  of  Vegetable  Saps  and  Juices,"  Chem. 
News,  Vol.  LXXXIV  (1901),  p.  234. 


86  Diffusion  and  Osmotic  Pressuke 

„   ,  -pi,,  -p.  Os.  Pressure  in 

Celery :  * r- Ft*  mm.  of  Hg. 

Green  stalk  and  leaf         -        -        -  -1.4°  12,842.48 

White  parts -0.75°  6,880 

Carrot: 

Leaf  and  stalk          -  -1.2°  11,007.84 

Koot    -                -1.0°  9,173.2 

Cabbage: 

Outer  leaf         -        -        -        -        -  -1.1"  10,090.52 

Heart  - -0.85°  7,797.22 

Apple,  fruit     -        -                 ...  -1.4-  12,842.48 

Pear,  fruit  -------  -1.75°  16,053.2 

From  the  evidence  at  hand  it  may  therefore  be  concluded 
that  there  is  great  variability  among  different  plants  with 
regard  to  the  particular  substances  called  into  requisition 
to  maintain  the  turgor  pressure.  There  seems  to  be  a  gen- 
eral tendency  for  these  to  be  of  an  organic  nature,  and  to 
possess  rather  complex  molecules.1 

VI.       THE    MAINTENANCE  OF   TURGIDITY   IN    SPITE    OF    PERMEA- 
BILITY TO  CERTAIN  SOLUTES 

It  might  be  supposed  that  the  fact  of  greater  or  less  per- 
meability of  the  protoplasm  to  various  solutes  would  lessen 
the  value  of  the  osmotic  explanation  of  the  phenomenon 
of  turgidity.  This,  however,  does  not  necessarily  follow. 
While  certain  substances  are  diffusing  in  and  out  of  a  cell, 
its  turgidity  may  be  maintained  by  the  presence  within  the 
vacuole  of  some  other  osmotic  substance  or  substances  to 
which  the  protoplast  is  impermeable,  or  very  slightly 
permeable.  It  is  probable  that  this  is  what  occurs  in  living 
plant  cells.  These  effective  osmotic  substances  are  usually 
of  the  nature  of  carbohydrates,  plant  acids,  and  mineral 
salts.  They  are  probably  secreted  into  the  vacuole  by  the 
activity  of  the  surrounding  protoplasm.  How  this  can  occur 
is  not  yet  known.     The  process  must  involve  movement  of 

1  Cf.  Pfeffee-Ewart,  Physiology  of  Plants,  Cambridge,  1900,  p.  141. 


TURGIDITY  87 


solute  particles  from  a  lower  concentration  to  a  higher, 
against  their  own  diffusion  tension.  It  may  be  that  these 
substances  are  freed  from  the  protoplasm  in  a  certain  form, 
and  that,  after  entering  the  vacuole,  they  polymerize  or 
change  their  nature  in  some  way,  according  to  special  con- 
ditions there  existing.  The  accumulation  of  many  substances 
within  the  vacuole  (e.  g.,  anilin  dyes1  and  caifein)  is  surely 
due  to  a  chemical  reaction  after  the  substance  has  passed  the 
protoplasmic  layers. 

VII.       EELATION  OF  TURGIDITY  TO  VITAL  ACTIVITY 

Only  because  of  the  existence  of  the  phenomenon  of  tur- 
gidity  has  the  plant  organism  been  able  to  develop  into  what 
it  is.  In  several  ways  turgidity  is  absolutely  essential,  and 
in  many  others  advantageous,  to  vital  activity  as  it  is  now 
exhibited  in  plants. 

a)  The  retention  of  form. — By  means  of  turgor  pressure 
the  delicate  fluid  or  semi-fluid  plasma  of  the  cell  is  kept 
pressed  out  against  the  cellulose  wall,  and  the  plasmic  mem- 
branes are  thus  kept  in  a  uniform  state  of  tension,  upon  which 
condition  some  of  their  physical  properties  doubtless  depend ; 
as  has  been  seen,  for  instance,  great  variations  in  turgor  may 
so  affect  the  protoplast  as  utterly  to  change  its  permeability 
to  certain  solutes. 

b)  Mechanical  support. — All  parenchymatous  tissues  and 
nearly  all  filamentous  and  uni-cellular  forms  owe  most  of 
whatever  rigidity  they  possess  to  the  stress  set  up  between 
the  internal  osmotic  pressure  and  the  elastic  force  of  the 
stretched  cell  walls.  A  heavy  weight  might  be  supported 
upon  a  pile  of  inflated  footballs  if  they  were  properly  placed, 
but  if  the  individual  balls  were  leaky  they  would  no  longer 
be  available  for  such  a  purpose.  In  a  similar  manner  the 
plasmolysis  of  any  thin-walled  tissue  is  accompanied  by  a 

ilbid.,  pp.  119  ff. 


88  Diffusion  and  Osmotic  Pressure 


more  or  less  marked  collapse.  Also,  the  existence  of  differ- 
ences in  the  turgor,  and  hence  in  the  tissue  tensions,  of  dif- 
ferent parts  of  the  body  in  higher  plants,  increases  the 
mechanical  strength  of  the  whole  structure  to  a  very  marked 
decree.  The  difference  in  tension  between  the  pith  and  cor- 
tex of  many  herbaceous  stems  is  an  illustration  of  this  fact. 

c)  Growth. — Exactly  what  the  relation  between  growth 
and  turgor  may  be  cannot  yet  be  stated,  but  there  is  good 
evidence  to  show  that  in  the  presence  of  turgidity  growth  is 
accelerated  and  in  its  absence  retarded.  Increase  in  thick- 
ness of  the  cell  wall  cannot  take  place  unless  the  protoplast 
is  kept  turgid,  and  thus  closely  applied  to  it,  as  was  shown 
by  Keinhardt.1  Other  evidence  along  this  line  is  that 
obtained  by  Klebs,2  when  he  succeeded  in  causing  a  new 
cellulose  wall  to  form  within  the  old  one  by  keeping  the  pro- 
toplast in  a  state  of  plasmolysis.  Also  the  work  of  Bower3 
needs  to  be  considered  here.  This  author  brings  out  very 
clearly  the  fact  that  there  is  a  close  connection  between  wall 
and  protoplasm,  by  a  study  of  the  strands  and  fibers  which 
remain  joining  the  protoplast  to  the  wall  in  a  plasmolyzed 
cell.  He  thinks  that  the  attachment  of  the  protoplasm,  which 
results  in  the  formation  of  these  strands,  is  closely  con- 
nected with  the  process  of  wall-formation.  The  strands  are 
not  usually  opposite  on  the  two  sides  of  a  common  wall,  and 
thus  apparently  have  no  relation  to  pores  through  the  wall. 

Experiments  on  the  effect  of  light  and  temperature  led 
Copeland4  to  the  conclusion  that  growth  regulates  turgor 
rather  than  turgor  growth. 

i  M.  O.  Reinhaedt,  u  Plasmolytische  Studien  zur  Kenntniss  des  Wachsthums 
der  Zellmembran,"  Festschrift  fur  Schwendener  (1899),  p.  425. 

2  G.  Klebs,  "  Beitrage  zur  Physiologie  der  Pflanzenzelle,"  Unters.  aus  d.  bot. 
Inst,  zu  Tubingen,  Vol.  II  (1888),  pp.  489-568. 

3  F.  O.  Bower,  "  On  Plasmolysis  and  its  Bearing  upon  the  Relations  between 
Cell-wall  and  Protoplasm,"  Quart.  Jour.  Microsc.  Sci.,  Vol.  XXIII  (1885),  pp.  157-457. 

*E.  B.  Copeland,  Ueber  d.  Einfluss  von  Licht  u.  Temperatur  auf  den  Turgor, 
Halle  a.  S.,  1896. 


TURGIDITY  89 


d)  Curvature. —  Since  most  plant  curvatures  are  due  to 
modified  growth,  it  is  to  be  expected  from  what  has  just 
been  said  that  turgor  must  play  an  important  part  in  these 
phenomena.  A  discussion  of  the  relations  of  this  question 
will  be  found  in  Pollock's  paper1  on  root  curvature.  To 
enter  into  this  much  mooted  question  would  be  going  too  far 
afield  from  the  present  subject. 

Other  more  rapid  curvatures  (such  as  those  of  the  pulvini 
of  the  leaves  of  the  Mimosa,  the  stamens  of  Berberis,  various 
tendrils,  etc.),  which  are  not  due  to  growth,  have  already 
been  mentioned.  They  owe  their  existence  entirely  to  turgor 
changes. 

e)  Work. —  Turgor  is  also  of  great  benefit  to  the  plant 
in  that  it  gives  it  a  means  of  doing  work,  of  overcoming 
resistance.  The  lifting  of  sidewalks  and  buildings  and  the 
splitting  of  cliffs  by  the  roots  of  trees  are  examples  of  the 
extent  to  which  this  pressure  may  be  developed.  For  it  must 
be  remembered  that  the  growing  region  of  any  plant  is 
always  mechanically  weak;  it  owes  practically  all  its  power 
of  resistance  to  the  turgidity  of  the  cells.  Pf effer 2  has  made 
many  tests  and  measurements  in  this  field,  and  Rodewald3 
has  applied  mathematics  to  the  problem,  showing  how 
osmotic  pressure  is  a  very  considerable  source  of  energy  to 
the  plant.  Of  course  the  energy  ultimately  comes  from  the 
heat  of  external  objects. 

VIII.       SUMMARY    OF    THE    CHAPTER 

Turgidity  is  the  immediate  result  of  osmotic  pressure  within 
the  cell.  It  arises  from  pressure  developed  within  the  cell 
sap  of  solutes  which  are  unable  to  penetrate  the  surrounding 

ij.  B.  Pollock,  "The  Mechanism  of  Root  Curvature,"  Bot.  Gaz.,  Vol.  XXIX 
(1900),  pp.  1-63. 

2  W.  Pfeffer,  "Druck  und  Arbeitsleistungdurch  wachsendo  Pfianzen,"  Abhandl. 
d.  k.  sdchs.  Ges.  d.  Wiss,  zu  Leipzig,  math.-physik.  Klasso,  Vol.  XX  (1893),  p.  285. 

3  H.  Rodewald,  "Ueber  die  durch  osmotische  Vorgauge  moglicke  Arboitsleistuug 
der  Pfianzen,"  Ber.  d.  deutsch.  bot.  Ges.,  Vol.  X  (1892),  pp.  83-93. 


90  Diffusion  and  Osmotic  Pkessure 


plasmatic  membranes  or  which  penetrate  them  very  slowly, 
the  concentration  (and  hence  the  pressure)  being  greater 
within  the  cell  than  in  the  external  solution.  The  internal 
concentration  is  probably  kept  up  by  the  chemical  activity 
of  the  protoplasm  itself,  substances  of  the  nature  of  soluble 
carbohydrates  or  organic  acids  being  formed  and  secreted 
into  the  vacuole.  How  such  a  movement  of  solutes  against 
the  direction  of  their  own  diffusion  tension  can  occur  is  not 
yet  explained.  Perhaps  they  change  their  nature  after  leav- 
ing the  protoplasmic  layer  and  entering  the  vacuole.  Turgor 
pressure  may  vary  in  different  cells  through  wide  limits 
(from  less  than  two  to  more  than  a  hundred  atmospheres) 
and  in  the  same  cell  the  variation  during  different  periods 
of  growth  may  be  almost  as  great.  Turgidity  is  influenced 
by  variations  in  the  amount  of  water  at  hand  and  also  by 
various  conditions  which  affect  the  permeability  of  the 
protoplasm  directly. 


CHAPTER  II 

ABSORPTION  AND  TRANSMISSION  OF  WATER 
I.     ABSOKPTION    OF  WATER 

As  has  been  seen  in  the  previous  chapter,  it  is  absolutely 
essential  that  every  living  mass  of  protoplasm  be  saturated 
with  water.  This  is  so  primarily  on  account  of  the  fact  that 
the  energy  transformations  which  are  designated  as  vital 
phenomena  occur  solely  in  aqueous  solutions.  It  is  also 
true  because  of  the  fact  that  water  is  actually  used  in  these 
transformations ;  it  is  chemically  combined  with  other  sub- 
stances to  form  carbohydrates,  proteids,  etc.  Therefore  it 
becomes  necessary  that  every  active  cell  be  not  only  satu- 
rated with  water,  but  also  that  it  be  in  connection  with  an 
external  supply  of  this  material.  Especially  is  this  so  where 
water  is  lost  by  evaporation.  It  is  possible,  of  course,  for  a 
plant  cell  to  become  hermetically  sealed  within  a  water-proof 
wall  (e.  g.,  fungus  spores,  etc.),  but  as  long  as  it  is  active 
and  growing  it  cannot  be  so  shut  off  from  the  outward  sup- 
ply of  water. 

If  the  cell  be  naked  and  immersed  in  water,  the  supply 
of  this  substance  is  always  at  hand  and  simply  diffuses  into 
the  protoplasm  as  it  is  used  in  metabolism.  If  the  organ- 
ism be  in  contact  with  a  moist  substratum  throughout  most 
of  its  surface,  as  is  the  Myxomycete  plasmodium,  the 
absorption  of  water  takes  place  from  the  imbibed  water  of 
the  substratum.  When  the  cells  are  surrounded  by  cellu- 
lose membranes,  these  are  kept  saturated  by  diffusion  from 
without,  and  the  protoplasm  absorbs  its  needed  water  from 
them.  The  cellulose  walls  of  ordinary  cells  act  like  the 
porous  and  imbibed  substratum  against  which  the  Myxomy- 

91 


92  Diffusion  and  Osmotic  Pressure 


cete  plasmodium  lies.  Thus,  if  the  plant  body  is  not  exten- 
sive and  is  mostly  in  contact  either  with  free  aqueous  solu- 
tion or  with  some  moist  substance,  the  problem  of  how  it 
obtains  water  is  a  simple  one.  Also,  mere  diffusion  from  one 
part  of  the  organism  to  another,  and  that  for  only  short  dis- 
tances, is  sufficient  to  account  for  all  the  water  transport  to 
be  met  with  in  such  plants. 

But  where  the  plant  body  elongates  upward  from  its  sub- 
stratum (a  phenomenon  occurring  in  all  forms,  from  the 
sporophores  of  fungi  to  the  stems  of  higher  plants),  it 
becomes  more  difficult  to  point  out  exactly  how  every  living 
cell  is  kept  saturated  with  water.  In  the  case  of  fungi  the 
rhizoidal  part  of  any  filament  is  in  direct  contact  with  the 
substratum,  and  here  the  solution  of  the  substratum  is  con- 
tinuous with  that  of  the  protoplasm,  through  the  saturated 
cellulose  membrane.  The  sporophore  is  in  connection  with 
absorbing  rhizoids  and  through  these  it  absorbs  not  only 
what  water  is  needed  for  growth,  metabolism,  etc.,  but  also 
what  is  needed  to  replace  that  lost  by  evaporation.  For  if 
the  loss  by  evaporation  is  not  made  up,  death,  or  at  least 
suspension  of  activity,  must  ultimately  ensue  from  desicca- 
tion. The  cellulose  walls  which  are  exposed  to  the  air  are 
more  or  less  thickened,  thus  causing  water  loss  by  evapora- 
tion to  be  much  less  pronounced  than  it  would  otherwise  be. 
Indeed,  fungi  are  seldom  found  growing  in  localities  where 
evaporation  is  very  marked.  In  the  case  of  algal  filaments, 
which  grow  upward  from  the  substratum,  and  in  that  of  ele- 
vated capsules  in  liverworts  and  mosses,  water  transport  is 
to  be  explained  in  the  same  way. 

In  the  higher  green  plants,  except  in  the  case  of  aquatic 
plants,  conditions  become  much  more  complex.  Here  the 
most  active  parts,  the  leaves  and  expanding  buds,  are  often 
removed  many  meters  from  the  soil  out  of  which  the  supply 
of  water   must   come.      In    these   plants   special   organs   of 


Absorption  and  Transmission  of  Water      93 


absorption,  the  roots,  and  a  special  region  of  transmission 
of  water,  the  xylem,  have  been  developed. 

But  the  great  expanse  of  exposed  surface  would  render  it 
utterly  impossible  that  the  living  cells  of  one  of  the  larger 
plants  be  kept  saturated  with  water,  were  it  not  for  the 
development  of  various  kinds  of  thickened  walls  and  even 
layers  of  dead  cells  (e.  g.,  cork)  covering  the  exposed  sur- 
faces. By  these  means  evaporation  is  greatly  reduced.  But 
to  carry  on  the  photosynthetic  process  it  is  essential  that 
carbon  dioxid  be  freely  absorbed  from  the  air.  Now  the 
only  manner  by  which  this  gas  can  reach  the  interior  of  the 
living  cells  is  by  passing  into  solution  in  the  imbibed  water 
of  the  cellulose  walls  and  then  diffusing  through  the  tissues 
as  a  solute.  The  complicated  structures  of  stomata  and  air 
chambers  bring  about  a  condition  of  things  such  that  thor- 
oughly saturated  cellulose  walls  are  exposed  to  the  air,  while 
at  the  same  time  a  minimum  amount  of  evaporation  is 
allowed  to  go  on.  The  plant  would  be  a  more  economical 
machine,  in  some  ways  at  least,  if  it  could  avoid  evapora- 
tion entirely,  but  this  is  impossible  if  imbibed  walls  are  to 
be  exposed  to  the  atmosphere.  By  far  the  greater  quantity 
of  the  water  absorbed  through  the  roots  finds  its  way  out  of 
the  green  plant  through  the  leaves,  and  is  of  no  direct 
material  use.  This  evaporation  plays  an  important  part  in 
keeping  the  green  parts  cool  when  they  are  subjected  to  the 
direct  rays  of  the  sun.  It  is  probable  also  that  much  of  the 
energy  for  raising  water  in  the  plant  comes  from  this  molecu- 
lar diffusion,  which  we  call  evaporation.  The  same  process 
produces  a  current  of  liquid  up  the  stem  and  thus  aids  in 
the  transmission  of  solutes. 

A  comparatively  small  amount  of  the  absorbed  water  is 
used  as  food  material  in  the  processes  of  photosynthesis, 
growth,  and  general  metabolism.  In  the  lower  forms  without 
chlorophyll  the  exposure  of  a  wet  membrane  to  the  atinos- 


94  Diffusion  and  Osmotic  Pressure 


phere  is  not  so  essential,  nor  is  it  essential  that  an  extensive 
surface  be  exposed  at  all.  A  comparatively  small  area  will 
suffice  for  the  evaporation  of  the  gaseous  products  of  respi- 
ration, etc.,  or  these  may  be  allowed  to  diffuse  outward  into 
the  solutions  of  the  substratum.  Thus,  in  colorless  parasites 
and  saprophytes  are  found  reduced  surfaces  covered  with 
leathery  tissues  which  contain  few  or  no  openings.  How- 
ever, water  is  so  plentiful  in  some  habitats  that  many  of  the 
forms  found  in  such  places  have  never  acquired  any  special 
protection  against  evaporation. 

In  aquatics  the  effects  of  evaporation  are  not  present; 
absorption  and  transmission  of  water  take  place  by  direct 
diffusion,  perhaps  mainly  through  the  roots,  however. 

Since  both  cell  wall  and  protoplasm  are  permeable  to 
water,  this  substance  will  diffuse  into  a  cell  when,  for  any 
reason,  its  diffusion  tension  is  less  within  than  it  is  outside. 
Thus,  if  the  surrounding  medium  is  a  very  dilute  solution, 
all  cells  must  absorb  water  until  an  equilibrium  is  estab- 
lished between  the  expanding  solutes  within  the  vacuole  and 
the  elastic  cellulose  wall  without.  By  imbibition  the  cell 
wall  is  kept  saturated  with  water  also,  so  that  there  is  direct 
water  communication  between  adjacent  cells  even  where 
there  is  no  protoplasmic  connection. 

This  form  of  water  absorption  is  universal  in  all  organ- 
isms. It  makes  no  difference  how  complex  the  form,  the 
individual  cells  stand  in  the  same  relation  to  water  external 
to  them  as  does  the  Myxomycete  plasmodium  moving  about 
in  its  moist  substratum.  Of  course  in  the  higher  forms  the 
cells  are  fixed  in  position.  In  the  latter  the  important 
special  condition  is  that  here  the  moist  substratum  in  which 
any  cell  lives  is  often  the  adjacent  living  tissue  of  the  same 
plant.  In  a  complex  tissue,  if  one  cell  loses  water  faster 
than  its  neighbors,  water  diffuses  from  them  into  it  and 
equilibrium  is  maintained. 


Absorption  and  Transmission  of  Water 


95 


In  all  organisms   except  the  very  lowest  the   power   to 
absorb  moisture  from  outside  the  body  is  possessed  by  only 
relatively  few  cells,  whose  external  position  fits  them  for  thN 
Ihus,some  submerged  aquatics  may  perhaps  absorb  equally 
throughout  the  whole  extent  of  their  comparatively  thinem 
dermis.    Partially  submerged  forms  can  absorb  only  through 
those  surfaces  which  are  under  water.    Ordinary  land  plants 
absorb  through  the  surface  layers  of  the  younger  portions  of 
their  roots,  the  surface  layer  being  often  greatly  extended  by 
the  development  of  root  hairs.     But  in  any  event,  no  matter 
where  the  water  passes  from  the  substratum  into  the  plant 
body,  absorption  always  takes  place  in  the  same  way      The 
cellulose  membranes  are  kept  wet  by  imbibition,  and  water 
diffuses  into  the  protoplasm  from  them.     The  forces  which 
cause  the  entrance  of  water  into  the  plant  are,  then,  partly 
those  of  adhesion  and  surface  tension,  and  partly  those  of 
simple  diffusion. 

That  the  rate  of  root  absorption  varies  with  the  tempera- 
ture of  the  soil  when  the  changes  in  temperature  are  grad- 
ual, as  was  demonstrated  by  Vesque,1  shows  that  this 
absorption  is  an  osmotic  phenomenon.  With  a  sudden  rise 
in  temperature  this  author  found  that  absorption  is  dimin- 
ished temporarily,  and  with  a  sudden  fall  it  is  augmented. 
This  is  probably  due  to  the  increased  pressure  of  the  inclosed 
gas  bubbles  at  higher  temperatures. 

II.       TRANSMISSION    OF    WATER 

a)  Water  loss.—  Within  the  single  cell  transmission  of 
water  occurs  mainly  by  simple  diffusion,  aided,  no  doubt,  by 
streaming  movements  of  the  protoplasm.  In  more  complex 
forms,  like  liverworts,  and  in  simple  tissue  masses  of  the 
higher   plants,  diffusion  from  cell  to  cell  through   the  Bat- 

l  J.  Vesque,  "De  l'influence  de  la  temperature  du  sol  sur  I'absorption  de  Feau 
paries  racmes," Ann.  set.  nat.  bot,  Ser.  VI,  Vol.  VI  (1877),  pp.  169-3 


96  Diffusion  and  Osmotic  Pressure 


urated  walls  brings  about  all  the  transfer  that  is  required. 
The  process  may  often  be  hastened  by  pits  and  protoplasmic 
connections.  In  higher  plants  phenomena  occur  which  are 
more  difficult  of  explanation.  In  order  to  discuss  the  trans- 
mission of  water  in  such  cases,  it  will  first  be  necessary  to 
consider  briefly  the  ways  in  which  water  is  lost  by  the  plant. 

(1)  Evaporation. —  As  has  been  stated  already,  by  far  the 
greater  part  of  the  water  absorbed  by  the  plant  is  lost  by 
evaporation.  Water  is  continually  evaporating  into  the  air- 
spaces of  the  plant  body  and  diffusing  out  into  the  external 
atmosphere  through  stomata  and  lenticels,  and  to  some  extent 
through  the  epidermis  itself.  The  vacuoles  of  the  leaf 
parenchyma  furnish  this  water  to  the  cell  walls,  and  thus 
their  solutions  become  more  concentrated  as  evaporation  con- 
tinues. Of  course  this  means  that  water  must  diffuse  into 
these  cells  from  cells  farther  back,  where  the  concentration 
is  not  as  great.  Eventually  these  cells  draw  water  osmotic- 
ally  from  the  xylem  strands.  The  source  of  energy  for  this 
process  of  concentration  of  leaf  solutions  is  the  heat  of  the 
surrounding  atmosphere,  which  causes  the  aqueous  molecules 
to  break  away  from  the  films  covering  the  parenchyma  walls 
next  to  the  air  chambers. 

(2)  Water  pores  and  nectaries. —  Water  pores  and  nec- 
taries are  curious  instances  of  the  passage  of  liquid  water 
out  of  the  plant  body.  No  completely  satisfactory  explana- 
tion of  these  occurrences  has  yet  been  given.  Osmotic  action 
surely  plays  an  important  part  here,  but  as  yet  no  accurate 
means  of  determining  the  exact  process  has  been  devised. 
The  difficulty  lies  in  the  fact  that  in  these  cells  the  move- 
ment of  the  water  is  in  the  opposite  direction  from  that  which 
would  be  expected  from  the  principles  of  osmotic  action.  The 
following  hypothesis  may  help  to  bring  the  facts  together  : 

The  modified  cells  bordering  a  water  pore   are  perhaps 
irritable  in  a  peculiar  way.    It  may  be  that  the  protoplasmic 


Absorption  and  Transmission  of  Water      (M 


sac,  upon  being  stretched  beyond  a  certain  limit,  changes  its 
nature  in  some  way  so  as  to  become  permeable  to  the  con- 
tained solutes.  If  this  were  the  case,  turgidity  would  rise  to 
the  critical  point,  and  then,  when  the  change  in  permeability 
took  place,  there  would  result  an  exudation  of  cell  sap  through 
the  plasmic  membrane,  the  contraction  of  the  previously 
stretched  cellulose  wall  forcing  the  solution  through  the 
now  permeable  layers.  This  exudation  might  be  equal 
in  all  directions,  or  might  take  place  mainly  in  the 
direction  of  the  water  pore,  as  if  the  portion  of  the  protoplast 
lying  next  the  modified  air  cavity  were  the  only  part  to 
become  permeable  at  the  assumed  critical  point.  It  seems 
probable  that,  if  such  an  occurrence  takes  place  at  all,  the 
change  is  brought  about  uniformly,  and  that  the  exudation 
from  the  vacuole  is  equal  in  all  directions.  But,  since  the 
adjacent  tissues  of  the  leaf  are  engorged  with  water,  a  marked 
flow  could  take  place  only  in  the  free  direction,  namely,  out- 
ward into  the  cavity  of  the  pore  itself. 

After  the  cellulose  walls  had  ceased  to  contract,  external 
pressure  would  be  removed  from  the  protoplast,  the  flow  of 
liquid  would  cease,  the  internal  pressure  would  have  fallen 
below  that  at  the  assumed  critical  point,  and  it  is  not  at  all 
inconceivable  that  under  these  conditions  the  protoplasm 
might  return  to  its  original  condition  of  semi-permeability 
toward  the  contained  solution.  If  this  were  so,  absorption 
from  the  surrounding  tissues  would  again  take  place,  and 
turgidity  would  gradually  return  until  the  critical  point  was 
again  reached,  when  the  process  of  external  discharge  would 
again  occur.  Evaporation  from  the  surface  of  the  exuded 
droplet,  which  must  become  rapid  as  soon  as  the  latter  is 
pressed  into  the  air  space,  would  concentrate  the  solution, 
and  thus  osmotically  prevent  resorption  of  the  extruded  water. 
More  than  that,  with  the  increasing  concentration  it  would  act 
as  a  plasmolyzing  solution  to  extract  still  more  water  from 


98  Diffusion  and  Osmotic  Pressure 


within.  Experiments  are  needed  to  determine  whether  or 
not  this  sort  of  extraction  of  water  does  really  take  place  in 
water  pores.  At  any  rate,  the  important  point  lies  here,  that 
there  is  an  original  exudation  of  cell  sap  through  the  proto- 
plasm. That  in  some  cases  at  least  the  exudation  is  truly  a 
portion  of  the  sap,  and  not  pure  water,  has  been  shown  by 
Bonnier1  in  the  case  of  honey-dew,  and  by  Dandeno2  in  the 
case  of  guttation  drops.  Also  Moll3  showed  that  when  shoots 
whose  leaves  bore  water  pores  were  injected  from  below  with 
the  juice  of  Phytolacca  decandra,  the  exudation  always  con- 
tained the  color.  Since  water  cannot  pass  from  the  xylem 
to  the  outside  without  traversing  the  cells  bordering  upon  the 
pore,  the  exuded  water  bearing  the  coloring  matter  must 
have  pased  through  these  cells. 

The  only  serious  difficulty  with  this  hypothesis  as  an 
explanation  of  the  action  of  water  pores  lies  in  the  assumed 
continually  decreasing  concentration  of  the  cell  sap.  But 
there  is  good  evidence  that  the  protoplasm  of  the  plant  cell 
is  continually  discharging  substances  into  the  vacuole,  which 
increase  the  osmotic  pressure  therein.  There  is  apparently 
no  reason  why  we  may  not  postulate  this  same  property,  per- 
haps in  an  unusually  marked  degree,  in  the  case  of  the  cells 
just  discussed. 

In  the  case  of  nectaries  there  is  exhibited  an  apparently 
similar  set  of  phenomena.  Wilson's  work 4  shows  that  after 
the  dissolved  substances  of  the  nectar  (mainly  sugar)  have 
once  passed  out  of  the  cells  and  into  the  cup  of  the  nectary, 

1  G.  Bonnier,  "  Recherches  experimentelles  sur  la  miellee,"  Rev.  gen.  bot.,  Vol. 
VIII  (1896),  pp.  1-22. 

2  J.  B.  Dandeno,  "An  Investigation  into  the  Effects  of  Water,  etc.,  on  Foliage 
Leaves,"  Trans.  Canad.  Inst.,  Vol.  VII  (1901),  pp.  238-350. 

3  J.  W.  Moll,  "  Untersuchungen  uber  Tropfenausscheidung  u.  Injection  von  Blat- 
tem,"  Verslagen  en  Mededeel.  d.  k.  Akad.  v.  Wetensch.  te  Amsterdam,  Vol.  XV  (1880), 
pp.  237-337. 

*  W.  P.  Wilson,  "  The  Cause  of  the  Excretion  of  Water  on  the  Surface  of  Nec- 
taries," Unters.  aus  d.  bot.  Bist.  zu  Tubingen,  Vol.  I  (1881),  pp.  1-22. 


Absokption  and  Transmission  of  Water      99 


the  high  osmotic  pressure  caused  by  evaporation  will  fully 
account  for  the  outward  passage  of  water  which  keeps  the 
nectar  of  flowers  and  leaves  in  the  liquid  condition  on  the 
dry  est  days.  Nectaries  may  even  be  artificially  formed  in  this 
way  ;  if  minute  granules  of  sugar  are  placed  upon  the  hyphse 
of  Mucor,  droplets  of  water  will  soon  form  and  increase  in  size 
until  they  run  down  and  fall  off.  The  same  may  be  done 
with  leaves ;  a  small  mass  of  sugar  upon  the  epidermis  will 
soon  extract  enough  water  osmotically  to  make  a  large  drop- 
let. The  method  of  extraction  of  the  water  in  these  arti- 
ficial cases  is  clear  enough  after  a  small  portion  of  the  sugar 
has  been  put  into  solution.  Water  to  form  the  first  minute 
amount  of  solution  cannot  be  withdrawn  from  the  leaf- 
cells  by  osmotic  action.  But  all  cellulose  membranes,  even 
cuticularized  ones,  are  more  or  less  saturated  with  water  of 
imbibition.  Now,  the  sugar  particles  resting  upon  the  leaf 
come  in  contact  with  these  moist  membranes  and  immedi- 
ately begin  to  dissolve  in  the  water  therein  imbibed.  After 
the  start  is  once  made,  be  it  ever  so  infinitesimal,  osmotic 
action  will  accomplish  the  outward  flow  of  water  from  the 
comparatively  weak  solution  within  the  cells  to  the  saturated 
one  on  the  surface. 

But  in  the  case  of  the  natural  nectary  the  original  exuda- 
tion of  the  sugar-containing  sap  has  not  yet  been  accounted 
for.  Perhaps  this  can  be  explained  in  a  manner  parallel  to 
that  just  postulated  for  the  case  of  the  water  pore.  At  a 
certain  stage  in  its  development  the  glandular  tissue  of  the  nec- 
tary may  undergo  a  change  such  that,  through  a  rapid  increase 
in  soluble  content,  the  osmotic  pressure  of  the  sap  of  its 
component  cells  rises  suddenly.  This  rise  in  turgor  pres- 
sure may  act  as  a  stimulus  upon  the  protoplasm,  and  the  latter 
may,  in  turn,  respond  by  a  change  in  its  structure,  so  that  it 
becomes  permeable  to  solutes  as  well  as  water.  If  this  be 
true,  the  contraction  of  the  stretched  cellulose  walls  must 


100  Diffusion  and  Osmotic  Pressure 

press  the  cell  sap  out  of  the  numerous  cells  of  the  gland, 
and  this  may  accumulate  on  the  external  surface.  Owing 
to  evaporation  this  droplet  of  exuded  sap  must  immediately 
be°in  to  increase  in  concentration,  and  from  that  time  on 
Wilson's  observations  are  sufficient  to  explain  the  maintenance 
of  liquid  in  the  nectary. 

It  has  been  shown,1  however,  in  certain  cases,  that  if  the 
nectar  be  removed,  new  nectar  will  be  secreted.  According  to 
the  above  hypothesis,  this  must  be  due  to  another  increase 
of  turgidity  and  another  exudation  of  cell  sap.  Thus  it  must 
be  supposed  that,  after  the  first  discharge,  the  conditions 
again  come  into  equilibrium,  and  the  plasmic  membranes 
again  become  semi-permeable,  only  to  repeat  the  former 
process  of  excretion  if  the  nectar  is  again  removed.  Per- 
haps the  evaporating,  and  therefore  concentrated,  solution 
on  the  surface  acts  as  a  plasmolyzing  agent,  keeping  the 
turgidity  of  the  gland  cells  down  by  extraction  of  water.  If 
this  were  so,  the  removal  of  the  nectar  might  easily  cause 
an  increase  in  turgidity,  which  might,  in  turn,  bring  about 
the  response  of  exudation. 

Wilson  found  the  shining  droplets  of  water  which  occur 
on  certain  molds  to  be  of  the  same  nature  as  the  artificial 
droplets  which  he  was  able  to  produce  by  sprinkling  the 
hyphse  with  pulverized  sugar.  When  these  natural  drop- 
lets are  removed  they  again  return,  but  this  does  not  occur 
if  the  hyphae  are  carefully  washed  with  distilled  water. 
Careful  examination  of  the  places  where  droplets  had  been 
removed  without  washing  showed  minute  crystals  of  sugar 
upon  the  surface.  Thus,  if  the  air  becomes  dry,  evapo- 
ration may  become  so  great  that  the  droplets  disappear,  but 
as  soon  as  evaporation  is  checked,  the  crystals  of  sugar  lying 
upon  the  surface  go  into  solution  in   the  imbibed  water  of 

i  H.  Haupt,  "  Zur  Secretionsmechanik  der  extrafloralen  Nektarien,M  Flora,  Vol. 
XC  (1902),  pp.  1-41.  On  the  general  subject  of  secretion,  see  W.  Pfeffee,  Studien  zur 
Energetik  der  Pflanzen,  Leipzig,  1892. 


Absorption  and  Transmission  of  Water    101 


the  cell  wall  and  cause  a  renewed  outward  flow  of  water,  and 
the  renewal  of  the  droplet. 

The  recent  work  of  Haupt  on  extra-floral  nectaries  adds 
to  our  knowledge  of  gland  action.  This  author  finds  that 
the  secretion  of  sugar  in  these  nectaries  begins  only  when 
the  gland  has  attained  a  certain  stage  of  development,  and 
then  only  when  transpiration  is  relatively  slight.  This 
makes  it  appear  as  though  the  protoplasmic  layers  become 
permeable  to  sugar  at  a  certain  phase  in  the  series  of  devel- 
opmental changes,  providing  there  be  at  that  time  a  great 
accumulation  of  water  in  the  cells.  This  last  provision 
means  that  the  cell  walls  and  plasmic  membranes  are  strongly 
stretched.  After  secretion  has  begun  an  increase  in  humid- 
ity causes  an  increase  in  the  excretion  of  water  from  the  gland, 
but  that  of  sugar  remains  constant.  Hence  it  may  be  con- 
cluded that  the  humidity,  i.e.,  the  amount  of  water  evapo- 
rated, has  no  direct  effect  upon  the  permeability  of  the 
protoplast  to  sugar.  For  the  beginning  of  the  secretion, 
Haupt  finds  that  a  certain  minimum  temperature  is  neces- 
sary. The  temperature  very  probably  affects  the  physical 
nature  of  the  protoplasmic  layer.  That  the  red-yellow  por- 
tion of  the  solar  spectrum  influences  the  activity  of  these 
glands  in  a  profound  manner  has  been  already  mentioned.1 

It  is  important  to  note  in  this  connection  that,  while  pro- 
toplasm is  generally  to  be  regarded  as  semi-permeable 
toward  many  solutes,  yet  there  is  evidence  from  almost  all 
parts  of  the  plant  kingdom  that  it  is  often  permeable  to  such 
substances  as  sugar,  acids,  inorganic  salts,  etc.  The  pene- 
tration of  these  substances  is  usually  a  slow  process,  how- 
ever. Now  the  variation  in  the  permeability  of  different 
protoplasts,  even  of  the  same  plant,  may  be  taken  as  evi- 
dence that  slight  differences  in  the  protoplasm  may  cause 
rather  great  differences  in  permeability.     This  makes  the 

i  See  p.  78. 


102  Diffusion  and  Osmotic  Pressure 

alteration  in  permeability  which  is  postulated  above  in  ref- 
erence to  water  pores  and  nectaries  less  difficult  of  supposi- 
tion than  it  might  appear  at  first  thought.  Also,  the  varia- 
tions in  permeability  which  are  known  to  occur  in  certain 
cells,  e.g.,  those  which  can  be  brought  about  in  almost  any 
cell  by  chemical  agents  like  Hg  Cl2,  and  by  change  in  tem- 
perature, for  example  the  cold  plasmolysis  of  Spirogyra, 
render  it  at  least  possible  that  excess  of  turgor  pressure  may 
alter  permeability.  Indeed,  a  similar  phenomenon  to  the  one 
postulated  was  observed  by  Oltmanns1  in  the  case  of  Fucus, 
and  it  is  more  than  probable  that  the  exudations  from  certain 
"sensitive"  pulvini  often,  if  not  always,  contain  solutes. 

(3)  Exudation. — If  the  stem  of  a  plant  be  severed  near 
its  base  and  a  mercury  manometer  be  attached  to  the  por- 
tion connected  with  the  roots,  an  exudation  of  water  under 
pressure  may  often  be  demonstrated.  This  so-called  exuda- 
tion pressure  is  not  uniform,  but  varies  in  an  irregular  man- 
ner, sometimes  sinking  below  the  zero  point  of  the  scale 
and  at  other  times  amounting  to  an  atmosphere  or  more.  A 
similar  pressure  is  reported  in  various  parts  of  the  plant, 
even  high  up  in  the  tops  of  trees,  as  was  noted  by  Molisch2 
in  the  case  of  Cocos  and  Arengo.  A  series  of  experiments 
upon  this  subject  was  performed  by  "Wieler,3  who  came 
to  the  conclusion  that  the  power  of  exuding  sap  is  a 
general  one  among  plant  cells.  Other  experiments  which 
are  not  nearly  so  satisfactory  are  those  made  by  Kraus*  and 

IF.  Oltmanns,  "  Ueber  d.  Bedeutung  der  Koncentrationsanderung  des  Meer- 
wassers  f iir  d.  Leben  d.  Algen,"  Sitzungsber.  d.  k.  preuss.  Akad.  d.  Wiss.  zu  Berlin, 
Jahrg.  1891,  p.  183. 

2H.  Molisch,  "Die  Secretion  des  Palmenweins  und  ihre  Ursachen,"  Oesterr. 
bot.  Zeitschr.,  Vol.  XLIX  (1899),  p.  74.  Also  see  W.  Figdok,  "Untersuchungen  uberd. 
Erscheinung  des  Blutungsdruckes  in  den  Tropen,"  Sitzungsber.  d.  kais.  Akad.  d. 
Wiss.  zu  Wien,  math.-nat.  hist.  Klasse,  Vol.  CVII  (1898),  p.  640.  This  is  noted  in 
Oesterr.  bot.  Zeitschr.,  Vol.  XLVIII  (1898),  pp.  359,  360. 

3A.  Wielee,  "  Das  Blnten  der  Pflanzen,"  Cohns  Beitrdge,  Vol.  VI  (1893),  pp. 
1-211. 

*C.  Keaus,  "Untersuchungen  iiber  den  Saftedruck  der  Pflanzen,"  Flora,  Vol. 
XL  (1882),  pp.  2  ff. 


Absorption  and  Transmission  of  Water    103 


Pitra.1      The    former    found    that    various    parts    of  stems 
and  roots  exuded  water  when  cut  out  from  the  plant  and 
placed  partly  submerged  in  wet  sand.  Details  of  the  experi- 
ments are    not  given,   and    there    are   many   circumstances 
in  the  somewhat  superficial  account  which  lead  the  reader 
to  doubt  whether  the  author  was   dealing  with   true  bleed- 
in^  •  for  there  are  other  factors,  such  as  the  expansion  of 
gases  in  the  wood,  which  may  cause  exudation  under  cer- 
tain conditions.     Pitra's  experiments  are  much  more  con- 
vincing   than    those    of    Kraus,  but    it     is    still    somewhat 
doubtful  whether  very  much  true  bleeding  was  observed  by 
either  of  these  authors.     Pitra  makes  one  observation  which 
is  of  interest  here,  however.     If  a  cut  shoot  be  inverted  with 
its  leaves  under  water  and  its  stem  in  air,  bleeding  from  the 
cut  surface  of  the  stem  will  ensue.     That  is,  if  leaves   are 
placed  in  a  position   to  absorb  water,  they  can  do  so  in  a 
manner  entirely  similar   to  that  exhibited  by  roots,  and  a 
leaf-pressure    corresponding   to   the    normal   root-pressure 
seems  to  be  developed.     The  observations  of  Pitra  in  this 
regard  have  been  substantiated  by  Molisch,2  who  finds  the 
same  to  hold  true  if  the  leaves  are  not  placed  in  water  but 
are  surrounded  by  moist  air. 

The  evidence  seems  to  be  good  that  bleeding  may  occur 
(1)  in  the  case  of  cut  stumps  to  which  active  roots  are  still 
attached ;  (2)  at  the  cut  surface  where  the  crown  or  inflor- 
escence has  been  removed  from  certain  palms  (Molisch),  and 
(3)  at  the  cut  surface  of  certain  stems  whose  leaves  are  sub- 
merged in  water  (Pitra).  It  may  occur  in  other  parts,  but 
it  seems  that  true  bleeding  has  not  been  unquestionably 
demonstrated  elsewhere  in  tall  plants. 

Exudation  pressure  has  often  been  ascribed  to  osmotic 

i  A  Pitra,  "Versuche  fiber  d.  Druckkraft  d.  Stammorgane  bei  d.  Erscheinun- 
gen  des  Blutens  U.  Thranens,"  Jahrb.f.  wiss.  Bot,  Vol.  XI  (1877),  pp.  43,-o.JU. 

2  H.  Molisch,  "  Ueber  localen  Blutungsdruck  und  seine  Ursachen,"  Bot.  Zatg., 
Vol.  LX  (1902),  pp.  45-63. 


104  Diffusion  and  Osmotic  Pressure 


phenomena  occurring  within  the  cells.1  If  this  be  the  true 
explanation,  we  have  no  exact  knowledge  of  the  processes  by 
which  these  phenomena  occur.  That  osmotic  pressure  within 
the  vacuoles  might  cause  movement  of  an  exuded  solution 
under  pressure,  through  intercellular  spaces  or  through  the 
channels  of  the  xylem,  need  not  be  questioned ;  this  can  be 
simulated  in  the  laboratory  with  the  ordinary  thistle  tube 
of  molasses  closed  with  animal  membrane.  But  to  explain 
the  phenomenon  of  exudation  pressure  it  must  be  shown 
how  it  comes  about  that  this  solution  gets  into  the  channels 
at  all.  A  small  amount  of  solution  might  be  exuded  in  a 
manner  similar  to  that  supposed  above  in  connection  with 
nectaries  and  water  pores.  But  the  absence  of  evaporation 
within  the  plant  body  will  deprive  us  of  that  source  of 
energy  to  which  has  been  ascribed  the  maintenance  of  a 
relatively  high  concentration  in  the  exudate  of  nectaries. 
However,  it  is  probable  that  even  in  nectaries  and  water 
pores  this  discovery  of  Wilson  is  not  of  fundamental  impor- 
tance. The  main  desideratum  is  to  know  how  the  original 
exudate  comes  to  get  through  the  otherwise  only  semi- 
permeable protoplasm ;  if  a  little  can  be  exuded  there  seems 
to  be  no  reason  why  more  could  not  pass  out  in  the  same 
way.  In  all  the  cases  of  exudation  the  exudate  is  known  to 
be  a  solution;  the  solutes  of  the  vacuoles  pass  out  and 
appear  in  the  exudate.  This  is  an  unquestionable  indica- 
tion that  the  protoplasmic  membrane  is  permeable  to  them. 
An  explanation  of  this  phenomenon  was  elaborated  by 
Pfeffer2  and  later,  apparently  independently,  by  Fuchs.3 
This  depends  upon  the  assumption  of  some  sort  of  vital 
activity  within  the  cells.  These  authors  point  out  that  in 
order  to  have  a  current  of  water  through  a  cell,  it  is  only 

i  W.  Pfeffer,  Osmotische  Untersuchungen,  Leipzig,  1877,  p.  223. 

2  Ibid.,  pp.  222-5. 

3  K.  Fuchs,  "Zur  Theorie  der  Bewegung  des  Wassers  imlebenden  Pflanzenkor- 
per,"  Beih.  Bot.  Centralbl.,  Vol.  X  (i901),  pp.  305-8. 


Absorption  and  Transmission  of  Water    105 

necessary  that  the  concentration  of  the  cell  sap  at  the  point 
of  entrance  be  higher  than  at  the  point  of  exit.  If  such  a 
condition  could  be  maintained,  a  slight  movement  of  water 
would  undoubtedly  take  place,  though  in  the  writer's 
judgment  it  would  be  very  inadequate  in  amount.  But  the 
insurmountable  difficulty  in  this  explanation  is  the  continu- 
ous maintenance  of  this  difference  of  concentration  in  differ- 
ent parts  of  the  same  cell.  This  could  only  be  accomplished 
by  the  active  absorption  and  precipitation  or  fixation  of  the 
active  osmotic  substances  in  one  region  of  the  protoplast, 
and  their  secretion  and  solution  in  another  part.  Such  a 
supposition  in  its  simplest  form  involves  the  carrying  back 
of  these  substances  by  the  protoplasm,  for  example,  from  one 
end  of  the  cell  to  the  other,  with  as  great  rapidity  as  they  can 
diffuse  in  the  opposite  direction  through  the  cell  sap.  This 
could  only  occur  with  enormous  expenditure  of  energy  on 
the  part  of  the  protoplasm,  and  it  is  difficult  to  imagine  any 
adequate  source  for  this  energy. 

Still  another  proposed  explanation  of  the  passage  of  water 
through  a  cell  in  any  given  direction  has  been  offered  by 
Pf effer.1  He  points  out  that,  if  the  membrane  where  the  water 
enters  be  less  permeable  to  solutes  than  that  where  it  escapes, 
a  continuous  flow  will  take  place.  This  is  undoubtedly  true, 
but  the  flow  would  be  still  more  marked  if  the  second  mem- 
brane were  removed  altogether;  for  it  can  act  only  as  an 
obstruction  to  the  upward  expansion  of  the  inclosed  liquid  as 
water  is  taken  in  through  the  semi-permeable  membrane 
below.  The  resulting  system  is  such  as  would  be  obtained  if 
the  thistle  tube  osmometer  used  for  illustration  of  osmotic 
pressure  were  to  have  its  stem  plugged  with  cotton.  The  cot- 
ton would  hinder  the  rise  of  the  liquid  column,  but  would  not 
stop  it   altogether.     Copeland2   has   actually  constructed  a 

i  W.  Pfeffer,  Osmotische  Untersuchungen,  Leipzig,  1877,  p.  225. 
2E.  B.  Copeland,  "An  Artificial  Endodermis  Cell,"  Bot.  Gaz.,  Vol.  XXIX  (1900), 
pp.  437-9. 


106  Diffusion  and  Osmotic  Pressure 


piece  of  apparatus  by  which  a  current  is  maintained  through 
a  cell,  different  parts  of  whose  wall  are  unequally  permeable. 

That  exudation  pressure  depends  upon  vital  activity  seems 
evident  from  the  fact  that  it  ceases  with  death.  Another 
line  of  evidence  which  points  toward  the  necessity  of  active 
protoplasts  for  exudation  is  that  brought  forward  by  Wieler,1 
when  he  records  that,  if  a  bleeding  plant  is  deprived  of  oxy- 
gen, exudation  stops.  He  observed  the  same  result  when 
the  plant  was  anaesthetized  with  chloroform.  The  exuded 
liquid  varies  in  its  concentration  in  different  plants,  usually 
becoming  weaker  as  bleeding  continues,  but  there  seems  to 
be  no  discovered  relation  between  the  exudation  pressure  and 
the  concentration  of  the  exudate.2 

The  whole  subject  of  exudation  and  sap  pressure  is  viewed 
in  an  entirely  new  light  by  Molisch3  in  his  last  paper  on 
these  phenomena.  He  presents  convincing  evidence  that  in 
all  cases  where  true  bleeding  has  been  observed  it  is  a  phe- 
nomenon connected  with  the  stimulus  of  wounding  or  with 
the  formation  of  new  tissue,  such  as  callus,  over  the  wound 
surface.  Thus,  the  method  of  decapitation  and  of  boring,  for 
the  study  of  exudation  pressure,  becomes  at  least  of  very 
doubtful  use  in  the  investigation  of  the  normal  pressure 
within  the  plant.  It  is  impossible  to  insert  a  manometer 
into  a  stem  without  making  a  wound,  and,  according  to 
Molisch's  conclusions,  this  wound  itself  is  sufficient  to  cause 
a  pathological  condition  of  the  neighboring  tissue  such  that 
exudation  ensues.  In  the  light  of  these  considerations,  then, 
it  seems  extremely  doubtful  whether  there  is  exhibited  in 
the  normal,  uninjured  plant  any  such  phenomenon  as  that  of 
sap  pressure.  Where  exudation  occurs  in  the  pathological 
wound  tissues  it  must  be  due  to  some  such  change  in  per- 
meability of  the  protoplasts  as  was  postulated  above. 

i  A.  Wieler,  "  Das  Bluten  der  Peahen,"  Cohns  Beitrage,  Vol.  VI  (1893),  p.  158. 
2  Ibid.,  pp.  65,  69. 

3H.  Molisch,  "Ueber  localen  Blutungsdruck  und  seine  Ursachen,"  Bot.  Zeitg., 
Vol.  LX  (1902),  pp.  45-63. 


Absokption  and  Transmission  of  Water    107 


(4)  Summary  of  ivater  loss. — By  evaporation  pure  water 
is  lost  from  the  plant.  Thus  the  osmotic  pressure  of  the 
fluids  in  the  leaves  and  near  evaporating  surfaces  is  increased 
and  other  water  diffuses  to  these  regions,  thus  tending  to 
re-establish  an  equilibrium  of  diffusion  tension.  Water  is 
eventually  taken  from  the  xylem  vessels  and,  since  these  are 
not  lined  with  protoplasm,  a  mass  movement  of  water  is  set 
up,  flowing  upward  through  the  xylem  strands.  This  carries 
with  it  whatever  dissolved  substances  have  been  extruded 
into  these  strands  from  the  roots.  Evaporation  cools  the 
leaves.  Leaves,  etc.,  are  able  to  extrude  solution  upon  their 
surface  through  specialized  openings,  the  glands  and  water 
pores.  The  exact  process  by  which  this  extrusion  takes  place 
is  not  known ;  it  is  probably  an  osmotic  one,  perhaps  coupled 
with  some  periodic  change  in  permeability  of  the  protoplasts. 
Leaf  cells  will  act  in  the  same  way  in  the  opposite  direction 
without  regard  to  water  pores  (Pitra,  Molisch). 

Root  cells  are  able  to  take  in  water  and  solutes  and  then 
to  pass  them  on  into  the  xylem.  It  seems  that  sap  pressure, 
whether  in  roots  or  in  wound  tissue,  must  be  explained  along 
the  same  general  lines  as  the  action  of  glands  and  water  pores. 
Whether  a  periodic  external  exudation  occurs  in  roots  is  not 
known,  but  it  seems  not  improbable  that  at  some  times  roots 
may  let  out  solutes.  Indeed,  Czapek1  and  Molisch2  have 
observed  extrusions  from  root  hairs  which  are  not  unlike 
those  from  water  pores.  It  may  be  that  some  sort  of  a  peri- 
odic extrusion  of  solutes  is  a  fundamental  property  of  proto- 
plasm. If  this  be  true,  it  is  a  phenomenon  not  unlike  that 
exhibited  in  pulsating  vacuoles. 

b)  The  upward  movement  of  ivater  in  trees  and  other 
tall  plants.  —  Owing  to  almost  insurmountable  difficulties  in 

IF.  Czapek,  "Zur  Lehre  von  den  Wurzelausscheidungen,"  Jahrb.  f.wiss.  Bot., 
Vol.  XXIX  (1896),  pp.  321-90. 

2  H.  Molisch,  "Ueber  Wurzelausscheidungen  und  deren  Einwirkung  auf  or^-a- 
nische  Substanzen,"  Sitzungsber.  d.  kais.  Akad.  d.  Wiss.  zu  Wien,  math.-nat.  hist. 
Klasse,  Vol.  XCVI  (1887),  pp.  84-109. 


108  Diffusion  and  Osmotic  Pressure 

experimentation,  an  exact  knowledge  of  the  manner  in  which 
water  is  raised  in  tall  stems  has  not  yet  been  reached. 
Various  hypothetical  explanations  of  the  observed  phenomena 
have  been  offered,  but  no  one  of  them  has  been  thoroughly 
established.  Imbibition,  capillarity,  the  lifting  power  of 
evaporation  exerted  upon  a  cohering  water  column,  physical 
osmosis,  and  undefined  "  vital  activity,"  have  all  been  invoked 
to  explain  the  phenomena  of  the  ascent  of  sap  in  trees.  It 
is  not  intended  to  take  up  here  the  discussion  of  any  of 
these  hypotheses  excepting  those  which  deal  with  osmotic 
pressure  and  diffusion. 

It  is  well  known  that  the  xylem  is  the  conducting  region 
for  water.  Since  the  trachese,  which  mainly  compose  it,  are 
dead  and  contain  no  protoplasmic  lining,  there  cannot  be 
attributed  to  them  any  active  part  in  lifting  the  water  which 
they  contain.  It  has  been  proposed1  as  a  partial  explana- 
tion of  this  rise  of  water  that  the  exudation  pressure  which 
is  made  externally  apparent  where  a  plant  is  cut  or  broken 
may  be  normally  active  within  the  xylem  and  may  thus  fur- 
nish a  part  of  the  needed  force.  This  is  a  very  plausible 
theory  if  not  pushed  too  far. 

It  can  hardly  be  supposed  that  if  this  pressure  is  con- 
cerned in  raising  water  in  the  xylem  it  is  exclusively  applied 
at  the  base  of  the  stem  or  in  the  roots.  It  is  much  easier  to 
suppose  that  the  various  groups  of  cells  exerting  exudation 
pressure  act  as  a  set  of  relay  pumps,  each  group  taking  the 

iThis  theory,  as  far  as  I  know,  was  first  clearly  put  by  Godlewski  in  his  paper 
entitled  "  Zur  Theorie  d.  Wasserbewegung  in  den  Pflanzen,'1  Jahrb.f.  wiss.  Bot.,  Vol. 
XV  (1884) ,  pp.  569-630.  Westerhaieb  had  somewhat  the  same  idea  a  year  previous  to 
this,  but  did  not  develop  it  as  well.  His  article  is  "  Zur  Kenntniss  der  osmotischen 
Leistungen  des  lebenden  Parenchyms,"  Ber.  d.  deutsch.  bot.  Ges., Vol.  I  (1883),  pp.  371-81. 
But  the  best  exponent  of  the  pumping  action  of  parenchyma  was  Jaxse,  whose  ideas 
are  expressed  in  a  paper  entitled  "  Die  Mitwirkung  des  Markstrahlen  beider  Wasser- 
bewegung  im  Holze,"  Jahrb.f.  tviss.  Bot,  Vol.  XVIII  (1887),  pp.  1-69.  The  last-named 
author  elaborated  Godlewskrs  theory  and  supported  it  with  experiment.  Many  more 
citations  might  be  made;  the  literature  is  very  voluminous.  For  a  very  complete  dis- 
cussion of  this  subject,  see  E.  B.  Copeland,  "  The  Rise  of  the  Transpiration  Stream," 
Bot.  Gaz.,  Vol.  XXXIV  (1902),  pp.  161-93,  260-83. 


Absorption  and  Transmission  of  Water    109 


sap  from  the  adjacent  tracheae  and  passing  it  on  upwards. 
But  if  sap  has  already  passed  through  a  set  of  these  active 
cells  in  a  manner  similar  to  that  described  in  connection 
with  water  pores,  its  concentration  cannot  be  lower  than 
that  of  the  cell  sap  of  these  cells  when  they  are  stretched  to 
their  utmost ;  indeed,  from  the  loss  of  pure  water  to  other 
cells  along  its  path  its  concentration  is  apt  to  be  even 
higher.  If  this  same  sap,  now  in  the  trachese,  is  to  enter 
another  set  of  such  active  cells  and  be  pressed  still  higher 
up,  the  sap  of  the  latter  must  necessarily  possess  a  higher 
osmotic  concentration  than  the  fluid  to  be  absorbed;  and 
after  it  has  been  pressed  out  of  them  into  tracheae  still 
farther  up  the  stem,  it  must  have  gained  in  concentration. 
Thus  any  such  explanation  of  the  rise  of  sap  in  stems 
involves  a  gradually  increasing  concentration  of  the  sap  as  it 
passes  upward.  There  seems  to  be  no  experimental  evidence 
of  this  as  a  fact.  It  is  true  that  the  sap  in  leaves  is  more 
concentrated  than  that  in  the  stem,  but  there  seem  to  have 
been  put  on  record  no  observations  of  a  gradual  increase  in 
concentration  toward  the  summit  of  a  tree.  Evaporation 
from  the  leaves  would  account  for  the  observed  fact. 

The  above  presentation  will  stand  for  various  hypotheses 
which  have  been  proposed  in  this  regard,  all  involving  some 
sort  of  a  periodic  variation  in  permeability.  Godlewski  and 
Janse  have  attempted  to  locate  the  active  cells  in  the  cortex 
or  medullary  rays  of  woody  stems,  but  these  attempts  have 
apparently  failed.  In  fact,  the  whole  idea  that  the  ascent  of 
sap  in  tall  stems  has  any  necessary  dependence  upon  the 
presence  of  living  cells  may  be  doubted  very  much  on  the 
following  experimental  grounds: 

In  his  study  of  bleeding  Wieler l  came  to  the  conclusion 
that  this  process,  while  it  is  probably  a  general  property  of 

i  A.  Wieler,  "Das  Bluten  der  Pflanzen,"  Cohns  Beitrage,  Vol.  VI  (1S93),  pp. 
1-211. 


110  Diffusion  and  Osmotic  Pressure 


protoplasm,  yet  plays  no  leading  role  in  the  lifting  of  water 
up  tall  stems.  As  far  back  as  1853  T.  Hartig1  showed  that 
a  poison  (ferric  pyrolignate)  would  pass  up  the  stem  of  a  tree 
for  over  12  meters.  He  bored  five  radial  auger-holes  in  the 
tree  trunk  near  its  base,  all  meeting  at  the  center.  These 
holes  were  filled  with  the  poison  solution  and  then  plugged. 
When  the  tree  was  cut  down,  the  star-shaped  stain  of  the 
poison  was  found  in  a  cross-section  over  12  meters  above  the 
holes.  Strasburger2  performed  the  same  experiment  more 
thoroughly.  Trees  were  cut  off  and  set  into  tubs  of  poison, 
such  as  aqueous  solutions  of  CuS04  and  picric  acid.  The 
poison  ascended  to  the  leaves,  a  distance  of  twenty-one  meters 
in  the  tallest  tree.  Of  course,  if  these  violent  protoplasmic 
poisons  ascend  the  trunk,  they  must  kill  all  cells  lying  in 
their  path.  Therefore  the  living  cells  of  the  stems  cannot  be 
necessary  for  the  rise  of  sap. 

But  after  the  leaves  had  been  killed  the  stem  ceased  to 
absorb  more  solution,  or  absorption  took  place  very  slowly. 
This  may  be  explained  by  the  fact  that  most  leaves  collapse 
and  dry  upon  being  killed.  The  cause  of  the  rise  of  sap  is 
perhaps  the  evaporation  from  the  surface  of  the  leaves,  and  in 
order  that  it  should  rise  the  leaves  must  be  in  their  normal 
turgid  condition.  Evaporation  may  thus  result  in  concen- 
tration of  the  solution  on  the  surface  of  the  walls  of  the 
parenchyma,  thus  causing  an  outward  osmotic  flow  of  water 
from  the  cells,  the  solutions  of  which  in  turn  become  more 
concentrated  and  extract  water  from  cells  lying  still  farther 
within  the  plant.  This  process  may  be  thought  of  as  con- 
tinuing until  water  is  finally  extracted  from  the  zylem. 
Here  the  osmotic  withdrawal  of  water  would  probably  become 

l  T.  Hartig,  "  Ueber  die  endosmotischen  Eigenschaften  der  Pflanzenhaute," 
Bot.  Zeitg.,  Vol.  XI  (1853),  pp.  309-17. 

2E.  Steasbuegee,  Ueber  den  Bau  und  die  Verrichtungen  der  Leitungsbahnen 
in  den  Pflanzen,  Jena,  1891;  idem,  "  Ueber  das  Saftsteigen,"  Histologische  Beitrage, 
Vol.  V,  Jena,  1893. 


Absorption  and  Transmission  of  Water    111 


a  mechanical  tension  upon  the  minute  films  and  columns 
reaching  to  the  foot  of  the  tree.  Strasburger's  experiments 
seem  to  show  that  atmospheric  pressure  cannot  play  a  part 
here ;  for  his  tallest  tree  was  as  high  as  a  column  of  water  which 
would  balance  a  pressure  of  two  atmospheres.  However,  it 
is  to  be  remembered  that  the  water  in  a  tree  trunk  is  not  in 
continuous  columns,  but  that  the  columns  are  divided  by 
air  bubbles. 

That  the  leaves  play  the  part  just  ascribed  to  them  is 
practically  proved.  Besides  the  experiments  of  Strasburger 
may  be  cited  that  of  Dixon,1  wherein  he  showed  that  when 
the  leaves  at  the  top  of  a  tall  shoot  were  killed,  the  upward 
passage  of  water  was  checked,  even  though  the  stem  were 
still  uninjured. 

The  hypothesis  that  evaporation  is  the  source  of  the 
energy  required  in  raising  water  dates  back,  in  its  general 
form,  to  Dutrochet.2  The  main  points  of  this  idea  are  the 
entrance  of  the  water  below  and  its  evaporation  above.  The 
point  which  has  caused  most  trouble  lies  in  the  lack  of  proof 
that  water  columns  such  as  are  found  in  the  tree  have 
cohesion  enough  to  be  drawn  up  by  evaporation  to  a  height 
far  exceeding  that  to  which  this  liquid  would  be  supported  by 
one  atmosphere.  That  it  will  cohere  somewhat  beyond  the 
height  to  balance  a  pressure  of  one  atmosphere  has  been 
shown  by  Bohm,3  Askenasy,4  and  Copeland.5  The  former 
of  these  showed  that  evaporation  from  the  surface  of  a  twig 
of  Thuya  attached  to  an  upright  tube  of  water,  the  lower  end 

1H.  H.  Dixon,  "Note  on  the  Role  of  Osmosis  in  Transpiration,"  Proceed.  Roy. 
Irish  Acad.,  Sec.  Ill,  Vol.  Ill  (1898),  pp.  767-75. 

2  M.  H.  Dutrochet,  M&moires  pour  servir  a  Vhistoire  anatomique  ct  physiolo- 
gique  des  v4gStaux  et  des  animaux,  Brussels,  1837. 

3  J.  Bohm,  u  Capillaritat  und  Saftsteigen,"  Ber.  d.  deutsch.  hot.  Ges.,  Vol.  XI 
(1893),  pp.  203-12. 

*E.  Askanasy,  "Beitrage  zur  Erklarung  des  Saftsteigens,"  Vcrhandl.  d.  naturl.- 
mel.  Verein  zu  Heidelberg,  N.  F.,  Vol.  V  (1896),  pp.  429-48. 

r'E.  B.  Copeland,  "The  Rise  of  the  Transpiration  Stream,"  Bot.  0a*.,  Vol. 
XXXIV  (1902),  pp.  161-93,  260-83. 


112  Diffusion  and  Osmotic  Pressure 

of  which  dipped  in  mercury,  was  sufficient  to  lift  a  column  of 
the  latter  liquid  higher  than  the  barometer  column  at  the 
time  of  the  experiment.  Askenasy  was  able  to  construct 
apparatus  whereby  he  could  demonstrate  a  pressure  of  90  cm. 
of  mercury  arising  from  evaporation  of  water  from  a  satu- 
rated plaster  of  Paris  plate.  Copeland  constructed  a  column 
of  plaster  of  Paris  3  mm.  in  diameter  and  12.4  m.  high, 
which  terminated  below  in  a  mercury  manometer  and  above 
in  a  Cu2  Fe(CN)6  membrane.  The  whole  column  was  as 
nearly  saturated  with  water  as  might  be  (there  were  many 
air-bubbles,  however),  and  the  membrane  above  was  covered 
by  an  exposed  solution  of  CuS04.  Evaporation  from  the 
surface  of  this  solution  caused,  in  five  days,  a  suction  on  the 
manometer  below  of  301  mm.  of  mercury.  Essentially  this 
apparatus  is  a  bundle  of  minute  water  columns  held  in  the 
pores  of  the  plaster,  but  broken  here  and  there  by  air  bubbles. 
There  was  certainly  enough  water  in  the  column  to  give  a 
pressure  of  more  than  459  mm.  of  mercury  (1  atmosphere 
minus  301  mm.).  If  this  be  true,  the  suction  set  up  by  evap- 
oration above  surpassed  the  pressure  of  an  atmosphere. 

This  theory  of  sap  ascent  seems  to  be  gaining  ground,  and 
it  is  quite  probable  that  the  idea  of  Godlewski  and  Janse 
will  eventually  be  put  entirely  aside.  The  experiments  of 
Strasburger  and  Dixon  show  that  osmotic  pressure  must  be 
active  in  the  leaves  in  order  that  sap  may  ascend  at  its  usual 
rate.  Another  proof  that  living  protoplasm  is  necessary  in 
leaves  lies  in  the  fact  that  transpiration  can  be  influenced  by 
anaesthetics.1     Such  reagents  act  in  a  similar  manner  to  the 

1  For  a  discussion  of  the  relation  of  anaesthetics  to  transpiration  see  H.  H. 
Dixon,  "On  the  Effects  of  Stimulation  and  Anaesthetic  Gases  on  Transpiration," 
Proceed.  Roy.  Irish  Acad.,  Ser.  Ill, Vol.  IV  (1898),pp.618-26;  H.^umelle,  "Influence  des 
anesthetiques  sur  la  transpiration  chlorophyllienne,"  Rev.  gen.  bot.,  Vol.  II  (1890), 
pp.  417-32;  idem,  "  Nouvelles  recherches  sur  l'assimilation  et  la  transpiration 
chlorophylliennes,"  ibid.,  Vol.  Ill  (1891),  pp.  241-88,  293-305 ;  A.  Schneider,  "The 
Influence  of  Anaesthetics  on  Plant  Transpiration,"  Bot.  Gaz.,  Vol.  XVIII  (1893),  pp. 
57-69;  A.  Woods,  "Some  Recent  Investigations  on  the  Evaporation  of  Water  from 
Plants,"  ibid.,  Vol.  XVIII  (1893),  p.  304-10. 


Absorption  and  Transmission  of  Water    113 


poisons  used  by  Strasburger,  though,  of  course,  in  not  so 
marked  a  manner.  They  probably  cause  partial  plasmolysis 
in  the  leaf  cells  and  thus  disturb  diffusion  and  evaporation. 
Still  another  point  which  may  be  construed  in  this  same 
manner  is  the  observation  of  Kossaroff '  that  an  increase  in 
the  amount  of  C02  in  the  water  in  which  are  placed  cut  twi^s, 
with  and  without  leaves,  is  accompanied  by  a  marked  falling 
off  in  water  absorption.  What  can  be  the  reason  for  this  we  are 
unable  even  to  conjecture,  but  it  certainly  appears  as  though 
the  chemical  nature  of  the  membranes  were  involved.  It 
cannot  be  due  to  the  osmotic  pressure  of  the  dissolved  C02, 
since  this  gas  penetrates  all  protoplasts  with  the  utmost  ease. 
Possibly  the  C02  may  precipitate  in  the  wood  and  form 
bubbles  which  plug  the  water  channels ;  but  this,  too,  seems 
unlikely.  It  seems  more  probable  that  the  dissolved  gas 
affects  the  protoplasmic  membranes  in  the  leaves,  causing 
some  change  in  their  osmotic  properties. 

The  whole  problem  of  water  ascent  remains  a  puzzling  one, 
one  which  must  wait  for  solution  until  the  development  of 
better  and  more  exact  methods  of  experimentation.  In  the 
meantime  it  will  probably  be  more  profitable  to  devote  atten- 
tion to  some  of  the  more  definite  and  restricted  problems  of 
the  cell  itself.  In  the  present  condition  of  our  knowledge  of 
plasmic  membranes,  for  example,  it  is  almost  foolhardy  to 
attempt  to  settle  such  a  complex  question  as  the  one  just  briefly 
reviewed ;  but  once  knowing  the  nature  of  these  plasmic  mem- 
branes, it  is  not  improbable  that  the  solution  of  the  problem 
of  water  transport  will  follow  as  the  simplest  corollary. 

III.       SUMMARY    OF    THE    CHAPTER 

In  general,  the  process  of  water  absorption  and  water 
movement  may  be  stated  as  follows  :  The  imbibed  water  of 
the  cell  walls,  the  water  of  the  protoplasm  itself,  and  the 

1  P.  Kossaroff,  "Die  Wirkuns?  der  Kohlens&ure  auf  den  Wassertransportin  den 
Pflanzen,"  Bot.  Centralbl.,  Vol.  LXXXIII  (1900),  pp.  138-44. 


114  Diffusion  and  Osmotic  Pressure 


water  of  the  substratum  in  which  the  organism  is  growing, 
are  to  be  regarded  as  one  continuous  mass  of  liquid.  Thus, 
if  the  diffusion  tension  of  water  in  any  part  of  the  plant 
becomes  less  than  it  is  at  any  other  point,  diffusion  takes 
place  and  equilibrium  is  restored.  In  the  same  way,  if  the 
diffusion  tension  within  the  plant  falls  below  that  of  the 
substratum,  diffusion  of  water  into  the  plant  must  imme- 
diately occur. 

This  process  of  simple  diffusion  is  sufficient  to  account 
for  absorption  and  for  transport  in  the  simple  plant  bodies 
and  in  any  small  portion  of  larger  bodies.  But  in  the  more 
complex  bodies  of  higher  plants  this  is  not  sufficient.  Just 
how  the  sap  is  raised  in  trees  is  not  surely  known.  There 
are  at  present  two  main  theories  to  account  for  it:  (1)  It  is 
supposed  to  be  raised  by  periodic  pumping  action  of  living 
cells  in  the  trunk.  (2)  It  is  supposed  that  evaporation  and 
the  resulting  osmotic  concentration  in  the  leaves  will  draw 
it  up  from  the  roots,  the  cohesion  of  the  minute  water 
columns  being  supposed  to  be  of  sufficient  magnitude  to 
prevent  their  being  broken  by  the  strain. 


CHAPTER  III 

ABSORPTION  AND  TRANSMISSION  OF  SOLUTES 

With  the  exception  of  the  naked  amoeboid  cells  occur- 
ring in  certain  stages  of  the  life  histories  of  a  few  fungi  and 
algse,  together  with  the  cell  complexes  constituting  Myxo- 
mycete  plasmodia,  plant  cells  are  unable  to  engulf  solid  food. 
The  presence  of  the  cellulose  membrane  makes  this  fact 
very  evident.     In  order  to  be  absorbed  into  a  cell,  any  sub- 
stance must  first  be  in  the   form   of  an   aqueous  solution. 
Even  where  the  process  of  engulfing  takes  place,  the  food 
does  not  truly  pass  inside  the  protoplasmic  body  until  it  is 
dissolved;    around  each  food  body  in  a  plasmodium  is    a 
vacuole  lined  by   a  plasmic  membrane  which    is  probably 
identical  in  nature  and  origin  with  that  covering  the  exte- 
rior of  the  protoplasmic  mass.     In  such  cases  the  food  is 
digested  in  this  vacuole  and  the  products  of  digestion  are 
absorbed  through  this  membrane. 

I.       ABSORPTION    OF    GASES 

There  are,  in  general,  two  forms  of  material  in  solution 
which  are  absorbed  by  the  plant,  namely,  gases  and  solids. 
As  has  been  seen,  gases  enter  into  aqueous  solution  when 
they  are  simply  brought  into  contact  with  the  solvent.  All 
the  natural  water  on  the  surface  of  the  earth  contains  in 
solution  oxygen,  nitrogen,  argon,  carbon  dioxid,  etc.  The 
first  and  the  last  of  these  gases  are  the  only  ones  which  are 
important  in  plant  metabolism.  The  moist  cellulose  mem- 
brane and  the  protoplasts  are  all  permeable  to  these  dis- 
solved gases,  and,  being  soluble  in  water,  they  will  diffuse 
wherever  it  can  diffuse.  Thus  there  must  be  a  tendency  to 
equalize  the  diffusion  tension  of  oxygen  and  carbon  dioxid 

115 


116  Diffusion  and  Osmotic  Pressure 


(and  of  the  other  two  gases  also,  though  they  are  not  used 
in  metabolism)  throughout  the  water  in  the  substratum  and 
that  contained  within  the  plant.  And,  since  these  two 
masses  of  water  are  continuous,  simple  diffusion  will  account 
for  the  exchanges  which  take  place  between  the  dissolved 
gases  of  the  soil  and  those  of  the  roots  of  the  land  plant. 
The  same  sort  of  diffusion  takes  place  between  the  internal 
solution  of  the  water  plant  and  the  surrounding  water.  Not 
only  may  absorption  thus  take  place,  but  also  the  giving  off 
of  gaseous  waste  products.  In  the  case  of  aquatic  sapro- 
phytes and  parasites,  oxygen  is  absorbed  and  carbon  dioxid 
given  off.  In  that  of  aquatic  green  plants  this  process  occurs 
in  darkness,  but  in  light,  carbon  dioxid  is  absorbed,  while 
oxygen  is  given  off.  The  roots  of  land  plants  are  always 
absorbing  oxygen  and  eliminating  carbon  dioxid. 

But  by  far  the  greater  portion  of  the  gaseous  exchange 
in  land  plants  and  semi-aquatics  takes  place,  not  through 
the  soil  water,  but  through  the  wet  membranes  which  are  in 
contact  with  the  air.  This  is  especially  so  in  green  plants, 
whose  leaves  are  peculiarly  constructed  so  as  to  expose  moist 
cellulose  membranes  in  air  chambers  which  are  in  connec- 
tion with  the  outer  air  through  stomata.1  By  a  number  of 
researches2  it  has  been  shown  that  dry  walls  are  but  slightly, 
if  at  all,  permeable  to  gases  and  that  the  more  moist  they 
are  the  more  readily  are  they  permeable. 

i  F.  F.  Blackman,  "  Experimental  Researches  on  Vegetable  Assimilation  and 
Respiration":  II,  "On  the  Paths  of  Gaseous  Exchange  between  Aerial  Leaves  and 
the  Atmosphere,"  Phil.  Trans.  Roy.  Soc,  London,  B.,  Vol.  CLXXXVI(1895),  pp.  503-62; 
H.  T.  Browne  and  F.  Escombe,  "  Static  Diffusion  of  Gases  and  Liquids  in  Relation 
to  the  Assimilation  of  Carbon  and  Translocation  in  Plants,"  ibid.,  Vol.  CXCIII 
(1900),  pp.  223-92. 

2N.  J.  C.  Muller,  "  Untersuchungen  uber  d.  Diffusion  atmospharischer  Gase  in 
derPflanzeundder  Gasausscheidungunter  verschiedenen  Beleuchtuugsbedingungen," 
Jahrb.f.  wiss.  Bot.,  Vol.  VII  (1869),  pp.  143-92;  E.  Lietzmaxn,  "  Ueberdie  Permeabili- 
tat  vegetabilischer  Zellmembranen  in  Bezug  auf  atmospharische  Luft,"  Flora,  Vol. 
LXX  (1887),  pp.  339-86;  J.  Wiesxer  and  H.  Molisch,  "Untersuchungen  uber  die 
Gasbewegung  in  der  Pflanze,"  Sitzungsber.  d.  kais.  Akad.  d.  Wiss.  zu  Wien  math.- 
nat.  hist.  Klasse,  Vol.  XCVIII  (1889),  Abth.  1;  P.  Clausen,  "  Ueber  die  Durch- 
lassigkeit  der  Tracheidenwande  fur  atmospharische  Luft,"  Flora,  Vol.  LXXXVIII 
(1901),  pp.  422-69. 


Absorption  and  Transmission  of  Solutes  117 


During  the  hours  of  sunlight,  when  the  process  of  pho- 
tosynthesis is  going  on,  carbon  dioxid  is  being  combined 
with  water  to  produce  carbohydrates  within  the  chlorophyll  - 
bearing  cells.  Thus,  at  this  time  the  diffusion  tension  of 
this  gas  in  the  solutions  of  these  cells  is  much  lower  than  it 
is  in  the  outer  air  and  in  the  air  chambers.  Therefore, 
there  must  be  a  constant  diffusion  stream  of  carbon  dioxid 
moving  through  the  stomata  into  the  air  chambers,  going 
into  solution  in  the  imbibed  water  of  the  moist  cell  walls 
wherever  it  touches  them,  and  diffusing  as  a  solute  through 
the  tissues  of  the  plant  to  the  places  of  low  diffusion  tension. 

The  oxygen  which  is  given  off  in  photosynthesis  finds 
its  way  to  the  outer  air  in  the  same  manner  as  that  by  which 
the  carbon  dioxid  enters.  The  oxygen  tension  becomes 
higher  in  the  green  cells  than  in  the  outer  air,  and  a  diffu- 
sion stream  of  this  gas  is  at  once  set  up  in  the  direction  of 
the  air  chamber,  where  it  goes  out  of  solution  and  then  dif- 
fuses as  a  gas  through  the  stomata  into  the  outer  air.  Dur- 
ing the  night  when  photosynthesis  has  ceased  this  oxygen 
stream  slackens  and  stops,  as  does  also  the  incoming  stream 
of  carbon  dioxid.  What  oxygen  is  used  in  respiration  at 
this  time  enters  from  the  outer  air,  and  the  carbon  dioxid 
produced  by  this  process  finds  its  way  out  in  a  manner 
exactly  similar  to  that  in  which  the  other  gas  escapes  during 
the  day. 

Epidermal  tissues  of  leaves  and  stems  are  also  imbibed 
with  water,  but  the  amount  of  water  which  they  can  hold  is 
comparatively  small  on  account  of  the  fact  that  the  external 
walls  are  heavily  impregnated  with  waxy  substances.  Hence, 
as  would  be  expected,  a  small  amount  of  gaseous  exchange 
between  the  atmosphere  and  the  plant  takes  place  directly 
through  these  membranes.1 

IF.  F.  Blackman,  "Experimental  Researches,"  etc.:  II,  "On  the  Paths  of 
Gaseous  Exchange  between  Aerial  Leaves  and  the  Atmosphere,"  Phil.  Traits.  Roy. 
Soc.,  London,  B.,  Vol.  CLXXXVI  (1895),  pp.  503-62. 


118  Diffusion  and  Osmotic  Pressure 


II.       ABSORPTION    OF    DISSOLVED     SOLIDS    AND    LIQUIDS 

It  is  probable  that  the  protoplasmic  membranes  of  plant 
cells  are,  normally  at  least,  slightly  permeable  to  all  sub- 
stances which  the  organism  needs  to  absorb.  We  have 
direct  evidence  of  the  permeability  of  protoplasm  to  many 
of  these  substances;  this  evidence  was  presented  in  chap,  i 
of  this  Part.  How  it  comes  about  that  a  cell  may  retain 
turgor  and  still  be  permeable  to  solutes  was  also  discussed 
there.  The  substances  which  are  diffusing  into  a  plant  cell 
at  any  moment  cannot  be  the  ones  which  are  producing  the 
turgor  pressure.  While  certain  organic  molecules  are  main- 
taining the  turgor,  for  instance,  many  inorganic  ions  may  be 
diffusing  into  the  cell,  because  the  partial  diffusion  tension 
due  to  them  is  lower  within  than  it  is  without.  It  is  also 
possible  that  permeability  changes  from  time  to  time,  so  that 
a  substance  which  cannot  penetrate  the  protoplast  at  one  time 
may  do  so  at  another.  Turgidity  is  maintained,  and  the 
protoplasmic  layer  kept  stretched  and  in  contact  with  the 
imbibed  cellulose  walls,  by  the  osmotic  pressure  of  certain 
substances  which  are  probably  formed  within  the  cell  and  to 
which  the  protoplast  is  but  slightly  permeable.1 

Thus  it  is  clear  that  the  absorption  of  solid  and  liquid 
solutes  from  the  surrounding  solution  is,  like  that  of  gases, 
merely  a  phenomenon  of  diffusion,  the  particles  moving 
toward  that  part  of  the  solution  where  lowest  diffusion  ten- 
sion of  that  substance  obtains.  Water  plants  possibly  absorb 
solutes  through  all  parts  of  their  submerged  surfaces.  Land 
plants  can  absorb  them  only  where  they  are  in  contact  with  the 
moist  substratum,  mainly  through  the  roots  and  root  hairs. 
The  so-called  power  of  selection  of  absorbing  organs 
deserves  some  attention  here.  It  is  observed  that  some 
plants  absorb  much  more  of  certain  substances  than  others, 

JO.  H.  VON  Mayenburg,  " Ldsungsconcentration  und  Turgorregulation  bei  den 
Schimmelpilzeu,"  Jahrb.f.  wiss.  Bot.,  Vol.  XXXVI  (1901),  pp.  381-420. 


Absorption  and  Transmission  of  Solutes  119 


and  that  different  plants  absorb  different  amounts  of  the 
same  substance.  Failure  to  absorb  a  solute  which  is  plenti- 
ful in  the  external  solution  may  be  due  to  either  one  of  two 
causes  :  either  the  protoplasmic  membrane  is  impermeable 
to  that  substance,  or  its  diffusion  tension  within  and  without 
are  equal.  If  a  substance  is  not  used  in  metabolism,  it  may 
simply  remain  in  the  cell  sap,  at  the  same  concentration  as 
is  the  surrounding  medium,  or  it  may  be  precipitated  or 
condensed  in  the  cell  sap  or  in  the  protoplasm,  and  thus  con- 
tinue to  accumulate.  An  example  of  this  last  possibility  is 
met  with  in  the  case  of  the  storage  of  carbohydrates.  Sugar 
diffuses  into  a  cell  and  is  there  polymerized  into  insoluble 
starch.  This  process  continually  removes  the  sugar  from 
solution,  so  that  inward  diffusion  must  continue  as  long  as 
starch  can  be  formed,  and  as  long  as  sugar  is  plentiful  out- 
side. Another  example  of  accumulation  is  the  case  cited  by 
MacDougal,  in  which  large  quantities  of  metallic  copper 
were  found  in  the  cells  of  an  oak  tree  (see  p.  69). 

If  the  substance  is  used  in  metabolism,  as  are  N03  ions, 
for  instance,  there  must  also  occur  a  continuous  diminution 
of  the  internal  diffusion  tension  of  these  particles,  which 
can  only  be  met  by  inward  diffusion  from  without.  If  a 
substance  is  being  rapidly  used,  this  inward  diffusion  will  be 
correspondingly  rapid ;  if  it  is  but  slowly  used,  absorption 
will  be  correspondingly  slow;  and  all  this  adjustment  of 
absorbing  power  may  thus  take  place  without  any  change  in 
the  permeability  of  the  plastic  membranes.  It  is  probable 
that  most  cases  of  selective  power  are  to  be  explained  in  this 
way.  Just  as  there  is  a  great  difference  in  the  use  of  the 
various  absorbed  solutes  by  different  plants,  so  also  there  must 
be  a  corresponding  difference  in  the  amounts  absorbed.  Thus, 
Demoussy,1  using  equivalent  quantities  of  several  salts,  found 

i  E.  Demoussy,  "  Absorption  elective  de  quelques  elements  mineraux  par  les 
plants,"  Compt.  rend.,  Vol.  CXXVII  (1900),  pp.  970-73;  cf.  P.  Bockget,  "  Sur 
Fabsorption  de  l'iode  par  les  vegetaux,"  ibid.,  Vol.  CXXIX  (1899),  pp.  7(38-70;  idem, 
same  title,  Bull.  Soc.  chim.  de  Paris,  Ser.  Ill,  Vol.  XXIII  (1899),  pp.  40,  41. 


120  Diffusion  and  Osmotic  Pressure 

that  KN03  was  taken  out  of  a  mixed  solution  two  to  six 
times  as  fast  as  NaN03  or  Ca(N03)2.  He  also  observed  that 
wheat  plants  absorb  K  ions  two  to  three  times  as  fast  as 
those  of  Ca  in  equivalent  solution.  On  the  other  hand, 
maize  absorbs  somewhat  more  Ca  than  K.  Pfeffer1  per- 
formed a  series  of  experiments  with  fungi,  which  gave  the 
general  result,  that  the  plant  takes  out  of  a  mixed  solution 
those  solutes  which  are  the  best  food  for  it.  These  would 
naturally  be  the  ones  whose  diffusion  tension  would  be  first 
to  diminish  in  the  active  cells. 

There  may  be  substances,  however,  such  as  certain 
poisons,2  which  react  upon  the  membranes  in  such  a  way  as 
to  cause  them  to  become  impermeable.  The  membranes 
must,  of  course,  be  more  or  less  permeable  to  such  sub- 
stances at  first,  else  they  could  not  react  upon  them.  But 
such  cases  are  very  rarely  met  with.  The  so-called  select- 
ive power  is  thus  probably  active  usually,  not  in  the  absorb- 
ing organs,  but  in  the  cells  where  the  substances  are  used  in 
metabolism. 

III.    TRANSMISSION    OF    SOLUTES 

An  internal  atmosphere  exists  in  the  plant,  occupying  the 
intercellular  spaces,  which  are  in  communication  throughout 
its  body  and  which  connect  with  the  lenticels  of  the  bark 
and  with  the  air  chambers  and  stomata  wherever  these  occur. 
By  means  of  this  internal  atmosphere,  gaseous  oxygen  may 
reach  the  more  deeply  lying  parts  of  the  body,  and  the 
gaseous  product  of  respiration  in  such  parts  may  find  its  way 
to  the  surface.  Thus,  it  is  probable  that  the  comparatively 
slow  process  of  hydro-diffusion  of  gases  is  replaced  by  the 
much  more  rapid  gas  diffusion  wherever  the  internal  atmos- 

1W.  Pfeffer,  "Ueber  Election  organischer  Nahrstoffe,"  Jahrb.f.  wiss.  Bot, 
Vol.  XXVIII  (1895),  pp.  205-68. 

2  Pulst  has  recently  shown  that,  while  copper  ions  are  readily  absorbed  by 
Mucor,  Aspergillus,  and  Botrytis,  they  are  not  taken  in  by  Penicillium.  See  C. 
Pulst,  "  Die  Widerstandsfahigkeit  einiger  Schimmelpilze  gegen  Metallgifte,"  Jah;  b. 
f.  wiss.  Bot.,  Vol.  XXXVII  (1902),  pp.  205-63. 


Absorption  and  Transmission  of  Solutes  121 


phere  makes  this  possible.  Mechanical  movements  of  the 
plant,  by  wind,  etc.,  probably  hasten  this  diffusion  by  creat- 
ing currents. 

Apart  from  this  gaseous  diffusion,  all  transmission  of 
solutes,  whether  gaseous,  solid,  or  liquid,  must  take  place  in 
the  form  of  aqueous  solution.  Diffusion  of  solutes  from  cell 
to  cell  takes  place  in  accordance  with  the  principles  of 
diffusion,  the  tendency  being  ever  to  equalize  the  diffusion 
tension  of  all  solutes  throughout  the  extent  of  the   solution. 

In  order  to  emphasize  the  fact  already  referred  to  that 
there  is  no  relation  between  the  turgidity  produced  by  one 
solute  and  the  diffusion  of  another  into  or  out  of  the  same 
turgid  cell,  the  following  example  may  be  taken:  Suppose 
an  artificial  cell  whose  membrane  is  impermeable  to  dissolved 
sugar  but  permeable  to  K  and  N03  ions.  Such  a  membrane 
may  be  made  from  copper  ferrocyanid.  Let  this  cell  be 
filled  with  a  solution  of  sugar  and  potassium  nitrate  so  made 
up  that  the  partial  pressures  of  the  two  substances  are  equal 
to  each  other,  say  five  atmospheres.  The  total  pressure  of 
the  solution  is  then  ten  atmospheres.  Now  let  this  cell  be 
placed  in  distilled  water.  Since  the  membrane  is  permeable  to 
KN03,  this  salt  will  immediately  begin  to  diffuse  outward, 
and  diffusion  will  continue  until  its  diffusion  tension  is  prac- 
tically as  great  outside  as  it  is  within  the  cell.  No  osmotic 
pressure  will  be  manifested  by  the  KN03,  excepting  the 
small  amount  due  to  friction  in  the  membrane.  But  the 
sugar  cannot  pass  out  of  the  cell,  and  must  therefore  exert 
its  full  pressure  upon  the  walls,  making  the  cell  turgid  and 
manifesting  a  stretching  force  of  nearly  five  atmospheres. 
Of  course,  as  the  cell  becomes  turgid  a  comparatively  small 
amount  of  water  will  enter  from  without,  and  thus  dilute 
the  internal  solution  of  sugar. 

So  far  the  illustration  shows  that  it  is  possible  for  turgidity 
to  be  maintained  while  a  substance  is  diffusing  out  of  the 


122  Diffusion  and  Osmotic  Pressure 


cell.  This  is  just  what  probably  occurs  during  the  trans- 
mission of  substances  from  one  cell  to  another  by  diffusion. 
After  a  time,  however,  the  artificial  cell  will  come  into 
equilibrium.  The  diffusion  tension  of  the  sugar  is  then 
just  equaled  by  the  resilience  of  the  walls,  and  that  of  the 
inclosed  KN03  by  the  diffusion  tension  of  the  same  salt  in 
the  surrounding  medium.  No  further  changes  of  concen- 
tration will  occur  until  some  alteration  is  made  in  the  con- 
ditions. Now  let  a  few  crystals  of  potassium  nitrate  be 
added  to  the  external  solution.  They  dissolve  immediately 
and  diffuse  equally  as  far  as  the  solution  extends.  But  now 
the  diffusion  tension  of  this  salt  has  been  raised  in  the  sur- 
rounding medium  while  it  remains  the  same  within  the  cell. 
However,  since  the  membrane  is  permeable  to  KN03,  this 
condition  cannot  last  long;  inward  diffusion  of  K  and  N03 
ions  will  soon  equalize  the  tension  within  and  without. 
Thus  it  is  shown  that  a  solute  may  diffuse  not  only  out  of 
but  also  into  a  cell,  the  latter  remaining  turgid  meanwhile, 
through  the  action  of  another  solute  to  which  the  osmotic 
membrane  is  impermeable. 

Mass  movements  of  the  sap  in  stems,  caused  by  changes 
in  temperature,  mechanical  bending  (as  by  the  wind),  etc., 
may  aid  very  much  in  keeping  the  various  solutes  equally  dis- 
tributed throughout  the  inclosed  solution.  The  mass  move- 
ment occasioned  in  the  solutions  by  evaporation  from  above 
(possibly  also  by  sap  pressure)  must  also  aid  in  this. 
Within  the  cells  the  streaming  movements  of  the  proto- 
plasm must  act  in  the  same  manner,  and  the  protoplasmic 
connections  between  adjacent  cells  probably  sometimes  set 
up  mass  currents  which  aid  in  the  transmission  of  solutes 
from  one  cell  to  another.  The  latter  consideration  is  prob- 
ably of  relatively  great  importance  in  the  case  of  the  trans- 
mission of  carbohydrates  and  other  plastic  materials  through 
the  phloem  region  of  stems.       But  by  far  the  most  impor- 


Absorption  and  Transmission  of  Solutes  123 


tant  factor  in  the  distribution  of  solutes  throughout  the 
plant  body,  whether  this  be  the  plasmodial  mass  of  a  Myxomy- 
cete  or  the  great  body  of  a  pine  tree,  is  probably  simple 
diffusion. 

If  it  were  not  for  the  phenomenon  of  turgidity,  the 
plasmic  membrane  would  not  be  in  condition  to  allow  dif- 
fusion to  take  place  readily.  But  the  membranes  do  not  di- 
rectly aid  in  the  transmission  of  solutes ;  they  only  hinder  it. 


CHAPTER  IV 

THE   INFLUENCE   OF   THE   OSMOTIC    PRESSURE   OF   THE 
SURROUNDING  MEDIUM  UPON  ORGANISMS 

I.       INTRODUCTORY 

Although  many  researches  have  been  carried  out  to 
determine  what  may  be  the  influence  upon  the  organism  of 
the  medium  in  which  it  is  grown,  it  is  only  within  the  last 
few  years  that  osmotic  pressure  has  been  investigated  in 
this  regard.  Most  experimenters  have  varied  the  chemical 
nature  of  the  medium  in  which  plants  and  animals  were 
grown,  and  have  argued  from  their  experiments  that  the 
presence  or  absence  of  certain  chemicals  brings  about  cer- 
tain effects  within  the  organism.  But  if  osmotic  pressure 
can  have  any  effect  upon  the  behavior  of  the  living  being — 
and  sufficient  evidence  has  now  been  accumulated  to  show 
that  it  does  have  a  very  marked  effect — then  the  results  of 
all  such  researches  must  be  considered  as  very  questionable. 
Nearly  all  the  published  accounts  of  the  influence  of  nutrient 
salts  upon  growth,  reproduction,  etc.,  in  plants  are  subject  to 
this  criticism,  that,  while  the  author  supposed  he  was  varying 
a  single  factor,  he  was  in  reality  varying  at  least  two.  Of 
course,  any  conclusions  reached  from  research  of  this  kind 
are  not  to  be  relied  upon. 

There  are  always  two  ways  in  which  a  nutrient  fluid  may 
affect  the  organisms  placed  in  it,  and  these  two  ways  corre- 
spond to  the  two  entirely  different  sets  of  properties  which 
are  possessed  by  every  solution.  The  solution  may  affect 
the  animal  or  plant  chemically,  on  account  of  its  chemical 
properties,  or  it  may  have  a  physical  effect,  on  account  of 
its  physical  properties.     Of  course,  since  both  sets  of  prop- 

124 


Influence  of  the  Medium  125 


erties  coexist  in  the  same  solution,  it  is  possible  and  prob- 
able that  the  organism  may  often  be  affected  in  both  ways 
at  the  same  time. 

By  the  chemical  properties  of  a  solution  are  meant  the 
chemical  nature  of  the  solute  or  solutes.  It  is  to  be  expected 
that  a  solution  of  cuprous  sulfate  will  affect  organic  beings 
differently  from  a  solution  of  cane  sugar  or  one  of  sulfuric 
acid;  these  solutions  are  chemically  very  different.  By 
physical  properties  are  meant  such  qualities  as  viscosity, 
transparency,  surface  tension,  osmotic  pressure,  etc.  The 
latter  is  the  only  one  of  these  which  it  is  necessary  to  con- 
sider here.  This  property  of  osmotic  pressure  has  been 
shown  to  be  of  general  importance  to  the  living  being 
grown  in  ordinary  nutrient  solutions,  but  it  has  long  been 
neglected  in  experiments  with  such  solutions.  Experi- 
menters with  nutrient  fluids  have  varied  the  chemical  nature 
of  their  solutions  without  taking  into  account  the  fact  that  in 
so  doing  they  were  very  probably  varying  the  osmotic  pres- 
sure also. 

When  these  workers  have  dealt  with  very  weak  solutions 
only,  it  is  evident  that  the  error  thus  introduced  is  practi- 
cally negligible  ;  the  osmotic  pressure  must  be  very  slight 
in  all  cases.  Thus  Ono1  showed  that  various  mineral  salts 
which  are  usually  considered  as  poisons  have  an  acceler- 
ating effect  upon  the  growth  of  certain  fungi  when  the 
solutions  are  very  dilute.  In  this  case  the  osmotic  pressure 
is  of  such  a  low  order  that  it  may  be  left  out  of  account. 
But  suppose  a  case  of  another  sort.  It  is  also  well  known 
that  a  stronger  solution  of  such  a  salt  as  cuprous  sulfate  will 
produce  almost  instant  death.  Shall  it  be  argued,  then, 
that  life  or  death  depends  upon  the  number  of  Cu  and  S04 
ions  which  may  penetrate  the  living  cells?     Or  shall  it  be 

i  N  Ono,  "  Ueber  die  Wachsthumsbeschleunigungeiniger  Algen  und  Pilz.ylurch 
chemische  Reize,"  Jour.  Coll.  Set.  Imp.  Univ.  T^o,  Vol  XIII  (1900),  Part  I:  B* 
Mag.,  Vol.  XV  (1900),  p.  75;  reviewed  in  Bot.  Gaz.,  Vol  XXX  (1900),  p.  4— 


126  Diffusion  and  Osmotic  Pressure 


argued  that  it  is  merely  a  question  of  osmotic  pressure  of 
the  solution,  and  that  the  chemical  nature  of  the  cuprous 
sulfate  has  nothing  to  do  with  the  response  ?  A  third 
possibility  is  to  conclude  that  both  these  factors,  always 
possessed  in  common  by  any  solution,  are  active  in  bringing 
about  the  observed  result.  Obviously  these  two  observa- 
tions, that  the  organism  lives  in  a  weak  solution  of  CuS04 
and  that  it  dies  in  a  stronger  one,  are  not  sufficient  to 
settle  the  question. 

There  are  several  different  ways  in  which  a  plant  cell 
may  be  affected  by  a  solution  into  which  it  is  plunged.  If 
the  solution  be  concentrated,  it  may  have  two  effects : 
(1)  Chemically,  the  solute  may  produce  a  response  in  the 
protoplasm  by  diffusing  into  it,  and  reacting  with  it  in  some 
way  as  yet  not  understood.  Thus,  the  effect  of  a  solution 
of  HgCl2  upon  plant  protoplasm  is  very  different  from  that 
of  cane  sugar.  (2)  Physically,  the  solution  may  affect  the 
cell  by  plasmolyzing  it,  or  partially  plasmolyzing  it,  or  by 
reducing  its  active  turgor  pressure.  It  has  been  seen  that 
this  effect  consists  primarily  in  extracting  water  from  the 
cell.  Secondarily,  it  results  in  an  increased  concentration  of 
the  contained  solution.  This  latter  may  again  result  in  a 
chemical  effect  upon  the  living  protoplasm,  but  of  this  we 
know  absolutely  nothing  as  yet.  If  the  solution  be  a  weak 
one,  its  physical  effects  will  be  just  the  reverse  of  those  just 
mentioned,  while  its  chemical  effects  will  often  be  the  same, 
but  perhaps  less  marked.  Physically,  it  will  allow  more 
water  to  diffuse  into  the  cell,  and  there  will  result  a  rise  in 
turgidity. 

In  order  to  answer  the  question  stated  above  with  regard 
to  the  nature  of  the  effect  produced  by  different  concentra- 
tions of  cuprous  sulfate,  experiments  upon  the  same  organism 
must  be  performed  with  other  salts  and  with  non-electrolytes, 
such  as  cane  sugar,  glucose,  etc.     In  making  these  solutions 


Influence  of  the  Medium  127 

extreme  care  must  be  used  to  have  them  of  exactly  the  same 
osmotic  concentration  as  those  of  CuS04,  which  were  pre- 
viously used.  Then,  if  the  organism  lives  in  all  the  dilute 
solutions,  no  matter  of  what  substance,  it  may  be  concluded 
that  the  determining  factor  is  one  of  osmotic  pressure.  If, 
on  the  other  hand,  the  organism  lives  in  the  concentrated 
solutions  of  cane  sugar,  glucose,  KN03,  NaCl,  etc.,  but  dies 
even  in  a  weak  solution  of  HgCl2  or  CuS04,  it  must  be 
concluded  that  the  conditions  of  the  medium  which  deter- 
mine life  or  death  are  of  a  chemical  nature.  It  is  thus  pos- 
sible to  analyze  the  effects  of  a  solution  by  using  a  number 
of  different  solutes. 

The  primary  effect  upon  an  organism  of  an  increase  in 
the  osmotic  concentration  of  the  surrounding  medium  is 
extraction  of  water,  that  is,  it  is  a  drying  effect.  The  pri- 
mary effect  of  a  decrease  in  the  osmotic  concentration  is  the 
reverse,  it  adds  water  to  the  organism. 

II.      PRESENTATION    OF    MATERIAL 

Following  is  a  review  of  the  several  lines  of  experimental 
evidence  which  have  been  brought  forward  in  connection 
with  the  question  of  the  effect  of  variations  in  the  concen- 
tration of  the  medium  upon  the  living  being.  Since  there 
is  so  little  to  be  presented,  the  work  upon  both  animals 
and  plants  will  be  included.  The  material  at  hand  will  be 
discussed  under  four  heads:  (1)  The  effect  upon  growth, 
(2)  the  effect  upon  reproduction,  (3)  the  effect  upon  move- 
ment, and  (4)  the  analogy  between  the  effects  of  high 
osmotic  pressure  of  the  medium  and  those  produced  by 
other  water-extracting  processes. 

a)  Variations  in  the  osmotic  pressure  of  the  surround- 
ing medium:  their  influence  upon  the  growth  and  form  of 
organisms. — A  number  of  observations  upon  various  organ- 
isms have  been  made,  all  tending  to  the  general  conclusion 


128  Diffusion  and  Osmotic  Pkessure 

that  growth  takes  place  more  slowly  in  a  concentrated  solu- 
tion than  in  a  weaker  one.  Loeb1  found  that  the  regenera- 
tion of  decapitated  tubularian  hydroids  occurs  much  more 
slowly  in  a  concentrated  than  in  a  dilute  solution.  The 
optimum  concentration  lies  considerably  below  the  normal 
concentration  of  sea-water,  in  which  these  animals  live  natu- 
rally. Similar  results  were  obtained  by  Yung,2  working  on 
tadpoles,  and  also  by  J.  L.  Frazeur  (with  annelids)  and  P.  E. 
Sargent  (with  Dero  vaga)  in  the  laboratory  of  C  B.  Daven- 
port.3 

The  first  of  a  very  important  series  of  observations  dealing 
with  the  effect  of  external  concentration  upon  cell  division 
was  made  by  Loeb4  when  he  discovered  the  fact  that,  in  fer- 
tilized Arbacia  eggs  which  were  placed  in  sea-water  whose 
concentration  had  been  raised  by  the  addition  of  NaCl,  the 
nuclei  divided  a  number  of  times  without  the  usual  accom- 
paniment of  the  segmentation  of  the  entire  egg.  When 
these  eggs  with  segmented  nuclei  were  returned  to  normal 
sea-water,  segmentation  of  the  cytoplasm  occurred  suddenly, 
the  number  of  segments  corresponding,  in  general,  to  the 
number  of  parts  into  which  the  original  nucleus  had  divided. 
Loeb  concludes  from  these  experiments  that  the  extraction 
of  water  by  high  osmotic  pressure  causes  a  falling  off  in  the 
irritability  of  the  protoplasm.  Whereas,  a  part  of  the  nor- 
mal process  of  cleavage,  namely,  that  pertaining  to  the 
nucleus,  is  carried  out,  the  remainder  of  it,  segmentation 
of  the  egg,  fails  to  occur  in  the  strong  solution.  The  cyto- 
plasm fails  to  perform  its  part,  although  the  nucleus  is  still 

1J.  Loeb,   Untersuchungen   zur  physiologischen  Morphologie   der  Thiere.  II: 
Organbildung  und  Wachsthum,  Wiirzburg,  1892. 

2  e.Yung,  "De  l'influence  des  variations  du  milieu  physico-chimique   sur  le 
developpement  des  animaux,"  Arch,  des  sci.  phys.  et  nat.,  Vol.  XIV  (1885),  pp.  502-22. 

3C.  B.  Davenport,  Experimental  Morphology,  New  York,  1899,  p.  365. 

*  J.  Loeb,    "Ueber    Kerntheilung  ohne    Zelltheilung,"  Arch.  f.  Entwickl.   d. 
Organismen,  Vol.  II  (1895) ,  pp.  298-300. 


Influence  of  the  Medium  129 


active.  Sperms  which  were  placed  in  the  strong  solution 
lost  their  irritability,  but  regained  it  upon  being  returned 
to  sea-water.  Another  instance  which  seems  to  show  the 
partial  loss  of  irritability  by  protoplasm  from  which  water  is 
osmotically  extracted,  is  stated  by  the  same  author  in  the 
same  article.  Hearts  of  ascidians,  crustaceans,  and  of 
embryo  and  adult  vertebrates  all  beat  less  rapidly  in  strong 
solutions  than  in  weak  ones. 

Morgan1  repeated  Loeb's  experiments  on  Arbacia  eggs 
with  practically  identical  results.  He  made  the  added 
observation  that  the  free  nuclear  division  described  above 
occurs  in  concentrated  solutions,  even  though  the  eggs  have 
not  been  fertilized.  This  author  also  gives  valuable  cyto- 
logical  notes  on  the  nature  of  the  free  nuclear  division. 

This  paper  by  Morgan  has  been  followed  by  several 
others  by  Loeb,2  the  results  of  which  may  be  brought 
together  as  follows  :  Unfertilized  eggs  of  Arbacia,  Strongy- 
locentrotus,  and  Asterias,  can  all  be  made  to  develop  parthe- 
nogenetically,  if  they  are  first  placed  for  a  time  in  sea-water, 
the  concentration  of  which  has  been  raised  by  addition  of 
either  an  electrolyte  (such  as  NaCl  or  MgCL)  or  a  non- 
electrolyte  (such  as  cane  sugar  or  urea).  They  must  then 
be  returned  to  normal  sea-water.  In  this  artificial  par- 
thenogenesis development  continues  until  the  animal  is  in 
the  Pluteus  stage.  This  is  as  far  as  the  development  of 
normally  fertilized  embryos  can  be  carried  in  aquaria.  The 
author  concludes  that  "  there  can  be  no  doubt  that  the  essen- 

i  T.  H.  Moegan,  "  The  Effect  of  Salt  Solutions  on  Unfertilized  Eggs  of  Arbacia," 
Science,  N.'  S.,  Vol.  VII  (1898),  p.  222.  \ 

2  J  Loeb  "On  the  Nature  of  the  Process  of  Fertilization  and  the  Artificial 
Production  of  Normal  Larva,  (Plutei)  from  the  Unfertilized  Eggs  of  the  Sea 
Urchin,"  Am.  Jour.  Physiol.,\oL  III  (1899),  pp.  135-8;  idem ■  » On  the  Artificial 
Production  of  Normal  Larvae  from  the  Eggs  of  the  Sea  Urchin  (Arbacia),  tfttd.. 
Vol  III  (1900),  pp.  434-71;  idem,  "On  Artificial  Parthenogenesis  in  Sea  Urchins, 
Science,  N.  S.,  Vol.  XI  (1900),  pp.  612-14;  idem,  "  Further  Notes  on  Artificial  Par- 
thenogenesis  and  the  Nature  of  the  Process  of  Fertilization,  Am.  Jour.  Physiol., 
Vol.  IV  (1900),  pp.  178-84;  idem,  "Artificial  Parthenogenesis  in  Annelids  (Chaetopte- 
rusj,"    Science,  N.  S.,  Vol.  XII  (1900),  p.  170. 


130  Diffusion  and  Osmotic  Pressure 


tial  feature  in  this  increase  in  the  osmotic  pressure  of  the 
surrounding  solution  is  a   loss  of  water  on  the  part  of  the 

egg" 

Furthermore,  the  same  author  showed  that  a  similar  arti- 
ficial parthenogenesis  may  be  brought  about  in  the  case  of 
Chretopterus,  a  marine  annelid.  Here  another  method  of 
treatment  would  also  bring  it  about,  namely,  a  slight  increase 
in  the  amount  of  potassium  in  the  medium  without  any 
increase  in  its  concentration.  This  is  termed  by  Loeb 
chemical  fertilization  in  contrast  with  physical  fertilization, 
the  form  described  above.  Chemical  fertilization  by  potas- 
sium is,  so  far,  impossible  in  the  eggs  of  Echinoderms.  Very 
recently  Loeb  and  Neilson1  have  shown  that  chemical  ferti- 
lization by  means  of  hydrogen  ions  is  possible  with  eggs  of 
Asterias,  and  the  same  sort  of  fertilization,  but  with  cal- 
ciumions,  was  brought  about  by  Loeb  and  Fischer1  in  the 
case  of  eggs  of  the  marine  annelid,  Amphitrite.  These 
phenomena  are  interesting  here  mainly  as  they  show  that  a 
chemical  influence  can  bring  about  the  same  effect  as  extrac- 
tion of  water. 

That  plants  grow  less  rapidly  in  concentrated  solutions 
than  in  more  dilute  ones  was  first  stated  by  Jarius,2  who 
worked  on  the  germination  of  seeds.  With  growing  plants, 
Stange3  showed  that  Pisum,  Phaseolus,  Lupinus,  etc., 
increase  in  thickness  more  rapidly  in  concentrated  solutions, 
while  more  rapid  growth  in  length  occurs  in  dilute  ones. 
He  suggests  that  the  effect  of  the  solution  may  be  different 
upon  the  meristematic  cells  of  the  growing   point  and  upon 

1  J.  Loeb,  M.  Fischer,  and  H.  Neilson,  "  Weitere  Versuche  uber  kttnstliche 
Parthenogenese,"  Vorlaufige  Mittheilung,  Pflugers  Arch.  f.  d.  ges.  Physiol.,  Vol. 
LXXXVII  (1901),  pp.  594-6. 

2M.  Jaeitjs,  "Ueberd.  Einwirkung  von  Salzlosungen  auf  d.  Keimungsprocess  d. 
Samen  einiger  einheimischer  Culturgewacb.se,"  Landwirtsch.  Versuchs-Stat.,  Vol. 
XXXII  (1886),  pp.  149-78. 

3  B.  Stange,  "  Beziehungen  zwischen  Substratconcentration,  Turgor  und  Wachs- 
turn  bei  einigen  phanerogamen  Pflanzen,"    Bot.  Zeitg.,  Vol.  L  (1892) ,  p.  253. 


Influence  of  the  Medium  131 


those  of  the  cambium.     Vandervelde1    experimented   upon 
the  germination  of  seeds  which  had  been  soaked  twenty-four 
hours  in  various  concentrations   of   several    salts.     As    the 
solution  became  stronger  the  number  of  seeds  to  germinate 
decreased,  but  after  a  certain  minimum  of  germination  was 
reached,  the  number  germinating    again    increased.      This 
author  suggests   that   the  failure  to  germinate  in  the  inter- 
mediate  concentration  is  due  to  the  penetration  of  the  salts, 
while  in  stronger  solutions   little  or  no  imbibition  of  water 
took  place,  and  the  seeds  when  planted  were  practically  the 
same  as  when  put  into  the  solution.     This   subject  has  been 
recently  taken  up   again  by  Buffum   and  Slosson.2     These 
authors  show   that  not   only  is  imbibition  of  seeds  greatly 
retarded   by  a  concentrated  solution  (as  was  known  before), 
but  also  germination  and  the  growth  of  the  plant  are  retarded 
in  the   same  manner.     This  is   true  without    regard  to  the 
chemical  nature  of  the  dissolved  substance.     Both  electro- 
lytes and  non-conductors  were  used.     Retardation  of  growth 
is  not  proportional  to  concentration,  however,  for  an  osmotic 
pressure  of  one  hundred  atmospheres   retards  growth  only 
about  twice  as  much  as  a  pressure  of  ten  atmospheres. 

Regarding  the  maximum  concentrations  in  which  fungus 
growth  can  occur,  investigations  have  been  made  by  Eschen- 
hagen  and  by  Raciborski.  Eschenhagen3  found  that  this 
maximum  was  different  for  different  fungi  studied,  but  the 
concentration  was  about  the  same  for  different  salts,  seeming 
to  show  that  it  was  a  purely  osmotic  effect.  For  Penicillium 
the  maximum  concentration  is  about  that  of  a   five-normal 

i  A.  J  J  Vandervelde,  "  Ueber  den  Eiafluss  des  chemischen  Reagentien  und 
des  Lichtes  auf  die  Keimung  der  Samen,"  Bot.  Centralbl,  Vol.  LXIX  (1897),  pp.  337-42. 

2  E  E  Slosson  and  B.  C.  Buffum,  "Alkali  Studies  II,"  Bulletin  39,  Wyoming 
Aaric  Exp  Sta.  (1898) ;  B.  C.  Buffum,  "Alkali  Studies  III,"  Ninth  Annual  Report, 
Wyoming  Agric.  Exp.  Sta.  (1899);  E.  E.  Slosson,  "Alkali  Studies  IV,"  ibid.  (1899); 
B.  C.  Buffum  and  E.  E.  Slosson,  "Alkali  Studies  V,"  Tenth  Annual  Report, 
Wyoming  Agric.  Exp.  Sta.  (1900) . 

3  F.  Eschenhagen,  Ueber  den  Einfluss  von  L6sungen  verschiedener  Koncentra- 
tionaufdas  Wachsthumvon  Schimmelpilze,  Stolp,  1889. 


132  Diffusion  and  Osmotic  Pressure 


cane-sugar  solution,  that  is,  about  111.5  atmospheres.  Raci- 
borski1  found  the  maximum  concentration  for  growth  of 
Basidiobolus  was  that  of  a  6  per  cent,  solution  of  NaCl,  or 
about  seventeen  atmospheres.  Yasuda2  published  an  account 
of  some  experiments  upon  infusoria  which  have  a  bearing 
here.  He  finds  that  these  organisms  are  able  to  adjust 
themselves  to  solutions  of  quite  high  concentration,  and 
that,  in  general,  the  limit  of  their  power  of  adjustment 
seems  to  be  at  about  the  same  osmotic  pressure,  no  matter 
what  salts  are  used.  In  other  words,  the  limit  to  adjust- 
ment is  apparently  an  osmotic  one  and  depends  upon  with- 
drawal of  water. 

The  experiments  of  the  present  author3  upon  the  physi- 
ology of  polymorphism  in  Stigeoclonium  need  to  be  con- 
sidered here.  In  the  stronger  solutions  (pressure  from 
323.7  cm.  to  647.4  cm.  Hg.)  this  alga  takes  the  form  of 
groups  of  spherical  cells  with  somewhat  gelatinous  walls. 
Multiplication  takes  place  rather  slowly,  cell  division  occur- 
ring in  all  directions  and  the  daughter-cells  immediately 
rounding  up  so  far  as  they  are  not  hindered  by  adjacent 
cells.  In  weak  solutions  (pressure  below  161.8  cm.  Hg.)  the 
behavior  is  entirely  different.  The  daughter-cells  elongate 
into  branching  filaments  composed  of  cylindrical  cells  and 
having  the  typical  appearance  of  Stigeoclonium.  Growth  is 
much  more  rapid  here  than  in  the  strong  solutions.  If  fila- 
ments are  transferred  to  a  strong  solution,  the  cells  round 
up  and  break  apart,  thus  producing  the  other  form. 

1  M.  Raciborski,  "  Ueber  den  Einfluss  Susserer  Bedingungen  auf  die  Wachs- 
thumsweise  des  Basidiobolus  ranarum,"  Flora,  Vol.  LXXXII  (1896),  pp.  107-32. 

2  Atsushi  Yasuda,  "Studien  uber  die  Anpassungsfahigkeit  einiger  Infusorien 
an  concentrirte  LOsungen,"  Jour.  Coll.  Sci.  Imp.  Univ.  Tokyo,  Vol.  XIII  (1900),  pp. 
101-40.    Reviewed  in  Bot.  Gaz.,  Vol.  XXX  (1900),  p.  285. 

3  B.  E.  Livingston,  (1)  "  On  the  Nature  of  the  Stimulus  Which  Causes  the 
Change  of  Form  in  Polymorphic  Green  Algse,"  Bot.  Gaz.,  Vol.  XXX  (1900),  pp.  289- 
317;  idem,  (2)  "  Further  Notes  on  the  Physiology  of  Polymorphism  in  Green  Algae," 
ibid.,  Vol.  XXXII  (1901),  pp.  292-302.  Some  parts  of  the  discussion  here  given  are 
quoted  from  these  articles. 


Influence  of  the  Medium  133 


In  the   first   series   of   cultures   several   modifications   of 
Knop's  solution  were  used.     This  solution  consists  of:  Ca 
(NO,),,  four  parts;     MgS04,  KN03,  and  K2HP04,  each 
one  part,  with   the   addition   of   a   trace   of   iron.    In  order 
to  determine  whether  a  change  in  the  concentration  of  this 
solution  would   affect   the  plant  in   a  chemical  or  physical 
way,  four  modified  solutions  were  made  up,  each  being  defi- 
cient in  one  of  the  four  constituent  salts.   The  deficient  salt 
was    reduced    to    one-tenth   its    normal   quantity,  and,  the 
decrease  in  osmotic  pressure  thus  brought  about  having  been 
calculated,  a  sufficient  amount  of  each  of  the  three  other 
salts  was  added  to  increase  the  pressure  by  an  amount  equal 
to  one-third  of  the  calculated  decrease.    Thus  were  obtained 
four  solutions,  all  of  which  had  the  same  osmotic  pressure, 
but  each  of  which  was  deficient  in  one  salt. 

The  calculations  for  the  pressure  corrections  were  made 
both  by  the  now  obsolete  method  of  De  Vries,  and  by  assum- 
ing that,  in  the  concentrations  used,  ionization  was  complete. 
Solutions  made  by  both  methods  gave  the  same  results  upon 
the  plant,  and  after  a  first  trial  the  second  method  of  cal- 
culation was  exclusively  used.     During  the  summer  of  1901 
the  pressure  of  nearly  all  these  solutions  was  tested  by  the 
freezing-point  method.     A  table  of  the  results  so  obtained 
will  be  found   in  the  second    paper  cited  on  this  subject. 
The  error  introduced  by  the  assumption  of  complete  ioni- 
zation   was    found    to    be    too    small  to  interfere    with  the 
accuracy  of   the   results    in    any    degree,   the    discrepancy 
between  the   real  and  the   calculated   pressures  lying   well 
within  the  limits  of  the  threshold  of   stimulation   for  this 

alga.  .  , 

The  cultures  showed  that  all  four  modified  solutions,  and 

the  normal  Knop's  solution  also,  influence  the  plant  in  exactly 
the  same  manner.  The  form  of  the  alga  is  always  deter- 
mined by  the  osmotic  concentration  of  the  medium,  and  is  not 
affected  by  the  varying  proportions  of  the  constituent  salts. 


134  Diffusion  and  Osmotic  Peessure 


In  the  second  series  of  cultures,  besides  the  normal  and 
modified  Knop's  solutions,  two  non-electroytes,  cane  sugar 
and  lactose,  were  used,  and  it  was  found  that  here  also 
the  concentration  is  the  controlling  factor  in  the  response  of 
the  plant.  It  must  be  noted,  however,  that  to  prevent  the 
formation  of  filaments  a  somewhat  higher  concentration  is 
required  of  these  sugars  than  of  the  inorganic  salt.  This 
is  perhaps  due  to  a  more  ready  absorption  of  the  sugars 
and  consequent  rise  in  internal  concentration.  Whatever 
may  be  the  cause  of  this  phenomenon,  it  seems  to  be  in  accord 
with  that  noted  by  van  Rysselberghe,1  namely,  that  cells  of 
Tradescantia,  etc.,  develop  a  greater  turgidity  in  salt  solu- 
tions than  in  those  of  sugar. 

It  is  thus  shown  conclusively  that  the  changes  in  the 
growth  of  this  plant  which  result  from  changes  in  the  con- 
centration of  the  medium  are  entirely  dependent  upon  its 
osmotic  pressure.  This  means  that  they  are  dependent  upon 
the  amount  of  water  contained  within  the  cells,  for  the  strong 
solutions  extract  water,  while  the  weak  ones  allow  it  to  be 
absorbed. 

In  a  weak  solution  vegetative  growth  is  very  much  more 
rapid  than  in  a  strong  one.  This  may  be  due  to  the  fact 
that  in  a  strong  solution  the  water  content  of  the  protoplasm 
is  reduced  in  amount  below  the  limit  for  optimum  lability. 
When  the  plant  grows  fastest  and  best  it  is  in  the  filamen- 
tous form.  In  the  weak  solution,  where  activity  seems  to  be 
at  a  maximum,  the  ions  of  the  electrolytes,  which  are  essen- 
tial for  metabolism,  are  not  plentiful.  This  may  suggest  how 
the  cylindrical  form  of  cell  with  its  increased  surface 2  may 

1  Van  Rysselberghe,  "  Reaction  osmotique  des  cellules  vegetales  a  la  concentra- 
tion du  milieu,"  Mem.  cour.,  pub.  par  l'Acad.  roy.  de  Belg.,  Vol.LVIII  (1898),  pp.  1-101. 

2  In  a  cylinder,  the  lateral  surface  is  greater  than  that  of  a  sphere  of  the  same 
volume,  as  long  as  the  ratio  of  the  length  to  the  diameter  equals  or  exceeds  2.727. 
In  typical  filament  cells  of  this  alga,  the  ratio  of  the  diameters  is  3,  and  it  is  often  4 
and  even  greater.  It  is  seldom  less  than  2.8.  Thus,  it  is  shown  that  the  filament  cell 
offers  more  surface  to  the  surrounding  medium  through  its  lateral  walls  alone  than 
does  the  palmella  cell  of  equal  volume. 


Influence  of  the  Medium  135 


be  advantageous.  At  any  rate,  we  may  be  sure  that  the 
greater  surface  of  the  cylinder  puts  the  plant  into  better  con- 
dition for  exchange  of  material  with  its  surrounding  medium. 
On  the  other  hand,  the  more  concentrated  solution  not  only 
withholds  water  from  the  cells,  but  presents  a  demand  upon 
them  for  water.  The  cell  meets  this  in  part  by  offering  as 
small  a  surface  as  possible  to  the  solution.  In  this  case, 
although  the  requisite  ions  may  be  present,  and  even  in  the 
right  number,  the  scarcity  of  water  in  the  protoplasm  may  so 
decrease  the  lability  that  rapid  growth  is  impossible.  We 
shall  see  that  there  is  also  a  corresponding  falling  off  in  the 
reproductive  activity  in  strong  solutions.  Perhaps  this 
response  is  attributable  to  the  increased  general  activity  in 
weak  solutions.  It  has  no  relation  to  the  form  of  the  cell, 
since  zoospores  are  produced  from  both  spherical  and  cylin- 
drical cells,  as  well  as  from  those  of  intermediate  shape. 

It  is  to  be  emphasized  that  in  the  stronger  solutions  cell 
division  and  growth  are  not  only  retarded,  but  the  direction 
of  the  dividing  planes  is  curiously  changed.  Whereas  in  the 
weak  solutions  the  cylindrical  cells  divide  only  by  walls  in 
one  direction,  the  spherical  cells  of  cultures  in  the  more  con- 
centrated solutions  divide  in  all  directions.  Whether  this  is 
due  to  the  change  in  form  of  the  cell,  or  directly  to  the  water 
content  of  the  protoplasm,  cannot  yet  be  decided. 

What  may  be  the  mechanics  of  the  rounding  up  of  cylin- 
drical cells  when  placed  in  a  concentrated  solution  is  one  of 
the  most  important  problems  suggested  by  this  research. 
The  fact  that  the  dead  cellulose  membrane  is  almost  entirely 
reshaped  during  this  process,  without  being  dissolved,  ren- 
ders it  probable  that  the  change  in  form  is  directly  caused  by 
some  change  in  turgidity  within  the  cell.  In  a  rounding  cell 
the  membrane  moves  and  changes  its  form,  and,  since  it  is 
entirely  inert,  the  source  of  this  motion  must  be  either  in  the 
activity  of  the  protoplasmic  body  itself,  or  it  must  be  in  the 


136  Diffusion  and  Osmotic  Pressure 


effective  turgor  pressure  of  the  mass  of  liquid  within.  But 
since  protoplasm  and  cellulose  wall  can  be  parted  so  readily 
during  plasmolysis,  the  first  alternative  is  well-nigh  untenable. 
If  the  wall  be  forced  into  the  spherical  shape  by  a  change  in  the 
pressure  from  within  this  must  be  brought  about  by  a  change 
in  the  volume  of  the  contained  liquids.  Now,  this  slight 
change  in  volume  which  might  produce  a  change  in  the  tur- 
gidity  of  the  cell  is  most  probably  due  to  an  alteration  in  the 
amount  of  cell  sap  within  the  vacuole.  When  the  surround- 
ing medium  suffers  change  in  concentration,  a  change  in  the 
volume  of  the  vacuole  may  come  about  through  the  proto- 
plasmic sac  either  secreting  liquid  or  acting  merely  as  a 
semi-permeable  membrane. 

When  filaments  are  placed  in  a  concentrated  solution 
their  behavior  suggests  at  once  partial  plasmolysis.  Water 
may  be  extracted,  the  effective  turgor  pressure  on  the  walls 
may  be  decreased,  and  by  the  forces  of  surface  tension  and 
cohesion  the  protoplasm  may  tend  to  round  itself  up  into  a 
sphere.  If  this  be  true,  we  have  an  explanation  of  the  lateral 
bulging  which  accompanies  the  longitudinal  shrinking  of  the 
cellulose  envelope.  If  the  protoplasm  tended  to  assume  a 
spherical  form  within  the  cylindrical  wall,  the  pressure  upon 
this  would  be  decreased  first  at  the  angles.  At  the  same  time, 
it  would  be  relatively  increased  upon  the  lateral  walls  near 
their  middle.  Thus  would  come  about  a  bulging  of  the  lateral 
walls  outward,  and  hence  a  shortening  of  the  cell  and  a  draw- 
ing of  the  end  walls  toward  each  other.  But  the  internal 
pressure  is  to  be  counted  as  almost  nothing  at  the  angles, 
while  it  is  still  considerable  in  the  middle  of  each  end  wall. 
So  the  margins  of  the  end  walls  would  approach  the  middle 
of  the  cell  more  rapidly  than  do  their  central  portions,  and 
splitting  of  the  common  membrane  of  two  adjacent  cells 
would  necessarily  ensue.  Several  facts  were  observed  in  the 
cultures  which  seem  to  support  some  such  hypothesis  as  the 


Influence  of  the  Medium  137 


one  just  stated.  I  have  placed  filaments  in  a  solution  where 
they  were  completely  plasmolyzed  and  killed,  without  any 
change  in  form.  In  solutions  a  little  less  concentrated  they 
are  not  plasmolyzed,  but  round  up  rapidly  and  soon  die, 
often  in  the  palmella  condition.  With  a  still  lower  pressure 
the  filament  cells  round  up  more  slowly  and  live.  Another 
fact  suggesting  this  idea  is  that  floating  filaments  can  resist 
a  stronger  solution,  and  can  resist  it  longer,  than  sunken 
ones.  The  former  are  to  some  extent  in  contact  with  the 
air,  and  thus  present  less  surface  than  the  latter  to  the  liquid. 
Still  another  observation  bearing  upon  this  hypothesis  of 
partial  plasmolysis  is  that  cylindrical  cells  are  the  only  ones 
which  are  able  to  change  their  form  after  they  have  become 
mature.  A  spherical  cell  must  remain  so  till  it  divides,  even 
if  it  be  in  a  solution  of  very  low  pressure. 

Raciborski1  made  what  must  be  regarded  as  essentially 
the  same  observation  as  the  one  just  discussed  upon  Basi- 
diobolus,  concerning  the  rounding  up  of  cells  and  the  change 
in  direction  of  cross  walls.  He  states  that  in  strong  solutions 
the  cells  became  rounded  and  separated  from  one  another, 
and  that  walls  formed  in  all  directions.  Although  he  paid 
little  attention  to  osmotic  phenomena,  yet  it  can  hardly  be 
doubted  that  Basidiobolus  ranarum  behaves  in  much  the 
same  way  as  does  Stigeoclonium. 

In  a  recent  paper  Beauverie2  has  described  some  interest- 
ing effects  of  the  osmotic  concentration  of  the  medium  upon 
fungi  and  higher  plants.  The  concentration  of  his  nutrient 
fluids  was  raised  by  the  addition  of  NaCl — a  very  question- 
able method,  especially  in  view  of  the  proof  brought  forward 
by   True3  that  this  salt  sometimes  has  a  poisonous  action. 

1  M.  Raciboeski,  "Ueber  d.  Einfluss  ausserer  Bedingungen  auf  d.  Wachsthums- 
weise  des  Basidiobolus  ranarum,"  Flora,  Vol.  LXXXII  (1896),  pp.  107-32. 

2  J.  Beauverie,  "Influence  de  la  pression  osmotique  du  milieu  sur  la  forme  et  la 
structure  des  vegetaux,"  Compt.  rend.,  Vol.  CXXXII  (1901),  pp.  226-29. 

3  R.  H.  True,  "  The  Physiological  Action  of  Certain  Plasmolyzing  Agents,"  Bot. 
Gaz.,  Vol.  XXVI  (1898),  pp.  407-16. 


■v 

138  Diffusion  and  Osmotic  Peessure 


Beauverie  found  that  fungus  hyphae  which  normally  grow 
upon  the  surface  of  the  nutrient  fluid,  and  even  rise  into  the 
air,  lose  this  habit  in  concentrated  solutions,  and  remain,  for 
the  most  part,  submerged.  Growth  continues  in  these  cases, 
but  seems  not  to  be  as  marked  as  in  weak  solutions.  Of  the 
hyphse  which  do  rise  into  the  air  from  the  concentrated 
medium,  the  cells  are  much  shorter  and  broader  than  those 
which  rise  from  a  weak  solution.  Details  are  not  given  in 
the  published  account,  but  apparently  we  have  here  a  very 
similar  response  to  the  one  which  was  obtained  in  the  case 
of  Stigeoclonium. 

The  same  author  has  also  experimented  upon  flowering 
plants,  e.  g.,  Pisum  and  Phaseolus.  Grown  in  a  strong 
solution,  the  stems  of  these  plants  are  short  and  thick,  and 
the  roots  show  a  remarkable  development  of  cork  tissue  on 
their  surfaces,  with  a  slight  development  of  pith.  He  also 
states  that,  while  in  weak  solutions  an  upward  bending  of 
the  roots  normally  occurs,  in  strong  solutions  these  grow 
vertically  downward.  The  upward  bending  in  weak  solutions 
has  been  ascribed  heretofore  to  aerotropism.  Perhaps  the 
extraction  of  water  which  occurs  in  the  strong  solution 
changes  the  irritability  of  the  roots  so  that  they  no  longer 
respond  normally  to  lack  of  oxygen.  In  the  stems  of  Pisum 
and  Phaseolus  is  perhaps  presented  another  case  of  cells 
failing  to  elongate  in  a  solution  which  extracts  water.  Much 
more  experimentation  is  needed,  however,  before  we  can  relate 
these  responses  in  higher  plants  with  those  of  algaB  and  fungi. 

b)  The  influence  of  external  concentration  upon  repro- 
duction.— Raciborski1  states  that  concentrated  solutions 
check  the  formation  of  zygospores  in  Basidiobolus.  In 
concentrated  solutions  Stigeoclonium2  failed  to  produce  any 

!M.  Raciborski,  "Ueber  d.  Einfluss  ausserer  Bedingungen  auf  d.  Wachsthums- 
weise  des  Basidiobolus  ranarum,"  Flora,  Vol.  LXXXII  (1896),  pp.  107-32. 

2B.  E.  Livingston,  (1)  "On  the  Nature  of  the  Stimulus  Which  Causes  the 
Change  of  Form  in  Polymorphic  Green  Algse,"  Bot.  Gaz.,  Vol.  XXX  (1900),  pp.  289- 
317;  idem,  (2)  "Further  Notes  on  the  Physiology  of  Polymorphism  in  Green  Algae," 
ibid.,  Vol.  XXXII  (1901),  pp.  292-302. 


Influence  of  the  Medium  139 


zoospores,  but  these  were  formed  in  great  numbers,  and 
very  rapidly,  in  the  weak  solutions.  Since  the  formation 
of  zoosporis  is  to  be  regarded  as  the  result  of  protoplasmic 
activity,  this  fact  is  added  evidence  that  the  cosmotic 
extraction  of  water  reduced  the  general  activity  of  the  pro- 
toplast. 

c)  TJie  influence  of  external  concentration  upon  irrita- 
bility. (1)  Changes  in  irritability. —  That  Loeb  observed  a 
loss  of  irritability  in  Echinoderm  sperms  when  these  were 
placed  in  a  concentrated  solution,  and  a  return  of  it  when 
they  were  brought  back  to  normal  sea-water,  has  already 
been  noted.  Eichter1  states  that  zoospores  of  Tetraspora 
lose  their  activity  in  strong  solutions,  but  regain  it  on  being 
returned  to  normal  sea-water.  The  writer  found  that  zoospores 
of  Stigeoclonium  lose  their  power  of  movement  in  concen- 
trated solutions.  Of  interest  here  is  also  the  observation  of 
Engelmann2  that  the  cilia  of  the  epithelial  cells  which  line 
the  frog's  oesophagus  become  much  more  active  in  pure 
water  or  a  very  weak  solution  than  in  a  solution  of  the  same 
concentration  as  the  fluids  of  the  animal's  body. 

Loeb3  gives  a  very  striking  account  of  the  reversal  of  a 
tropism  by  osmotic  extraction  of  water.  At  ordinary  tem- 
peratures the  larvse  of  Polygordius  and  certain  Copepods 
are  partly  positively  and  partly  negatively  heliotropic. 
Above  25°  C.  they  all  react  negatively,  while  below  10°  C. 
the  response  is  reversed,  and  they  all  become  positively 
heliotropic.  If  NaCl  is  added  to  the  normal  sea-water  in 
which  these  animals  are  living,  they  all  react  positively  to 
light ;  if  distilled  water  is  added,  they  all  react  negatively. 

1  A.  Richter,  "  Ueber  die  Anpassung  der  Sftsswasseralgen  an  KochsalzlOsungen  " 
Flora,  Vol.  L  (1892),  pp.  4-56. 

2T.  W.  Engelmann,  "  Ueber  die  Flimmerbewegung,"  Jena  Zeitschr. ,  Vol  IV 
(1S68),  pp.  321-479. 

3  J.  Loeb,  "  Ueber  kunstliche  Umwandlung  positiv  heliotropischer  Thiere  in 
negativ  heliotropische  und  umgekehrt,"  Pflilgers  Arch.f.  d.  ges.  Physiol.,  Vol.  LIV 
(1893),  pp.  81-107;  idem,  Physiology  of  the  Brain,  New  York,  1900,  p.  198. 


140  Diffusion  and  Osmotic  Pressure 


A  similar  reversal  of  tropism,  in  this  case  of  geotaxis,  was 
observed  in  Chromulina  woroniniana  by  Massart.1  Thus,  by 
osmotically  changing  the  amount  of  water  in  the  protoplasm 
the  irritability  of  these  organisms  can  be  reversed. 

(2)  Osmotaxis. — The  concentration  of  the  medium  acts 
as  a  directing  stimulus  upon  the  motions  of  certain  free- 
swimming  organisms.  This  form  of  response  has  been 
named  osmotaxis,  in  analogy  to  other  similar  responses  to 
light,  heat,  chemicals,  etc.  An  organism  is  said  to  be  posi- 
tively osmotactic  when  it  swims  from  the  weaker  to  the 
stronger  solution  where  these  are  brought  into  contact.  It  is 
negatively  osmotactic  when  it  swims  in  the  opposite  direction. 

Since  the  effect  of  high  concentration  of  the  medium  is 
to  extract  water  from  the  cell,  it  will  be  seen  that  there  must 
be  an  identity  of  nature  between  this  response  and  that  of 
hydrotropism.  An  organism  is  positively  hydrotropic  when 
it  bends  away  from  a  dryer  and  toward  a  moister  atmos- 
phere. This  phenomenon  is  exhibited  in  roots,  fungus 
-sporophores,  etc.  It  corresponds  to  negative  osmotaxis, 
in  which  the  organism  swims  from  a  region  where  water  is 
extracted  from  its  body  to  one  where  absorption  can  take 
place  more  freely.  Since  the  conditions  under  which  the 
two  responses  are  made  manifest  are  so  very  different,  it  is 
probably  well  to  retain  the  word  "osmotaxis." 

Rothert2  has  recently  devoted  an  article  to  the  discussion 
of  this  subject.  The  following  facts  are  mainly  derived 
from  this  source,  Stahl3  showed  that  Myxomycete  plas- 
modia,  which  had  become  accustomed  to  a  certain  concen- 
tration, would  be  repelled  by  any  other  concentration,  either 
higher  or  lower.     They  are  thus  negatively  osmotactic. 

1  J.  Massart,  "  La  sensibility  a  la  concentration  chez  les  fitres  unicellulaires 
marins,"  Bull.  deVacad.  roy.  de  Belgique,  Ser.  Ill,  Vol.  XXII  (1891),  pp.  148-67. 

2  W.  Rothert,  "  Beobachtnngen  rind  Betrachtungen  fiber  taktische  Reizerschei- 
nungen,"  Flora,  Vol.  LXXXVIII  (1901),  pp.  371-421. 

3E.  Stahl,  uZur  Biologie  der  Myxomyceten,"  Bot.  Zeitg.,  Vol.  XLII  (1884),  pp. 
145  ff. 


Influence  of  the  Medium  141 


Massart,1  experimenting  upon  certain  bacteria,  found 
that  they  were  negatively  osmotactic  to  solutions  of  many 
different  substances.  The  proof  that  the  phenomenon  is 
osmotaxis  and  not  chemotaxis  lies  in  the  fact  that  the  organ- 
isms  were  repelled  always  at  the  same  osmotic  concentration, 
irrespective  of  the  chemical  nature  of  the  solute.  Rothert 
found  Treptomonas  agilis  positively  osmotactic  toward  solu- 
tions which  are  so  concentrated  that  they  kill  the  organism 
by  plasmolysis.  Only  such  solutes  are  available  for  experi- 
ments upon  osmotaxis  as  are  known  to  be  unable  to  pene- 
trate the  protoplasm  of  the  organism  to  be  tested.  Of  course, 
if  penetration  occurs,  the  difference  in  concentration  within 
and  without  the  cell  can  last  but  a  short  time ;  it  will  soon 
be  equalized  by  the  inward  diffusion  of  the  solute. 

d)  The  analogy  between  the  effects  of  high  osmotic  pres- 
sure of  the  medium  and  those  produced  by  other  water- 
extracting  processes. — Attention  has  already  been  called  to 
the  fact  that  a  lowering  in  temperature  is  often  accompanied 
by  giving  out  of  water.  Thus  Spirogyra  filaments  when 
cooled  in  olive  oil  may  be  seen  to  give  off  water  before  freez- 
ing. It  seems  probable  that  in  this  case  the  protoplasm 
becomes  more  permeable  at  these  low  temperatures  and  thus 
the  solute  escapes  with  the  solvent.  If  this  is  true,  we  can- 
not look  upon  cold  plasmolysis  as  producing  a  concentration 
of  the  cell  sap.     It  would  only  decrease  its  volume. 

Of  course,  a  part  of  the  shrinkage  in  such  a  case  could 
be  accounted  for  by  the  diminution  of  osmotic  pressure  due 
to  cooling.     If  the  original  internal  pressure  were  p  at  t°C, 

then   it   would  decrease    to  p  ~o7q    ,    ,  at  t'°Q.     The  pres- 
sure of  the  external  solution  will  decrease  according  to  the 

1  J.  Massart,  "  La  sensibility  a  la  concentration  chez  les  ©tres  unicellulaires 
marins,"  Bull,  de  Vacad.  roy.  de  Belgique,  Ser.  Ill,  Vol.  XXII  (1891),  pp.  148-67 ;  idem, 
"Sensibilite  et  adaptation  des  organismes  h  la  concentration  dee  solutions  salines," 
Arch,  de  biol.,  Vol.  IX  (1899),  pp.  515-70. 


142  Diffusion  and  Osmotic  Pressure 


same  principle,  but  since  its  original  pressure,  say  s,  was 
much  smaller  than  p,  its  decrease  for  the  same  fall  of  tem- 
perature will  not  be  so  great  as  that  of  the  internal  solution. 
This  may  be  shown   thus: 


t'p  t's 

> 


273  -f*      273  +  £  ' 
when  P  >  s  . 

Thus,  the  internal  and  external  osmotic  pressure  will  be 
more  nearly  the  same  at  a  low  temperature  than  at  a  higher 
one.     The  two  pressures  should  become  equal  at  absolute  zero. 

No  measurements  have  been  made  to  determine  whether 
the  decrease  in  volume  of  the  Spirogyra  vacuole  is  propor- 
tional to  the  approach  of  the  external  and  internal  concen- 
trations toward  each  other.  This  should  not  be  a  difficult 
thing  to  settle.  But,  as  has  already  been  stated  (page  75), 
there  is  cryoscopic  evidence  that  the  extruded  liquid  is  not 
pure  water. 

The  identity  of  the  responses  obtained  by  Loeb  with 
Copepods  and  Polygordius  larvae  when  these  were  subjected 
to  cold  and  to  high  concentrations,  has  also  been  noted 
(page  139).  A  similar  change  of  tropism  occurs  among 
those  plant  lice  which  exist  in  two  forms,  one  winged  and 
the  other  wingless.  The  growth  of  wings  in  the  wingless 
form  can  be  called  forth  either  by  low  temperature  or  by 
allowing  the  plants  upon  which  the  animals  are  feeding  to 
dry,  thus  depriving  the  latter  of  water.  While  in  the  wingless 
condition  these  lice  are  negatively  heliotropic,  but  upon  devel- 
oping wings  they  become  positively  so.  Here  is  a  reversal 
of  tropism  brought  about  by  withdrawal  of  water,  but  this 
experiment  also  shows  that,  although  the  general  protoplas- 
mic activity  may  be  depressed  by  this  treatment,  yet  certain 
special  activities  (e.  g.,  those  involved  in  wing  formation) 
may  be  accelerated. 


Influence  of  the  Medium  143 


It  is  generally  known  that  lowering  of  the  temperature 
of  an  animal  heart  causes  the  beating  to  become  less  rapid. 
This  is  perfectly  parallel  to  the  falling  off  in  heart  activity 
in  strong  solutions,  as  observed  by  Miss  Shively  and  recorded 
by  Loeb  (page  129). 

In  my  own  experiments  on  Stigeoclonium,1  it  was  found 
that  the  organism  responds  to  drying  on  a  porous  plate  in 
exactly  the  same  way  as  it  does  to  change  from  a  weak  to  a 
strong  solution. 

Recently,  Greeley2  has  shown  that  by  cooling  Stentor 
ccerulcus  the  same  cessation  of  activity  and  rounding  up 
was  brought  about  as  when  the  animals  were  subjected 
to  the  action  of  concentrated  solutions.  However,  the 
effect  of  the  solution  was  not  reversible,  for  the  animals 
could  not  be  revived.  The  same  author  has  shown  that 
cold  plasmolysis  in  Spirogyra  is  reversible,  that  a  rise  in 
temperature  brings  the  plasmolyzed  alga  back  to  its  normal 

condition. 

During  the  summer  of  1901  Greeley3  was  able  to  pro- 
duce artificial  parthenogenesis  of  Echinoderm  eggs  by 
merely  keeping  them  for  a  time  at  a  low  temperature.  In 
these  cold-fertilized  eggs,  development  went  as  far  as  in 
normally  fertilized  ones  under  artificial  conditions. 

In  general,  then,  it  may  be  concluded  that  there  is  a 
striking  analogy  between  the  responses  obtained  in  these 
various  organisms  by  treating  them  with  strong  solutions 
and  by  extracting  water  from  them  in  any  other  way.  How 
much  further  we  may  go  in  this,  remains  for  future  experi- 
ment to  show. 

i  B.  E.  Livingston,  "  Further  Notes  on  the  Physiology  of  Polymorphism  in  Green 
Algae,"  Bot.  Gaz.,  Vol.  XXXII  (1901),  pp.  292-302. 

2  A  W  Greeley, -On  the  Analogy  between  the  Effects  of  Loss  of  Water  and 
Lowering  of  Temperature,"  Am.  Jour.  Physiol.,  Vol.  VI  (1901),  pp.  122-8. 

3 A  W  Greeley,  "Artificial  Parthenogenesis  Produced  by  a  Lowering  of  the 
Temperature,"  Am.  Jour.  Physiol.,  Vol.  VI  (1902),  pp.  296-304. 


144  Diffusion  and  Osmotic  Pressure 


III.       SUMMARY    OF    THE    CHAPTER 

As  far  as  investigation  has  gone,  it  has  been  found  that 
growth  is  accelerated  in  weak  solutions  and  retarded  in  con- 
centrated ones.     The  term  "growth"  here  includes,  not  only 
enlargement,  but  also  the  process  of  cell  division.       Also,  in 
some  cases  at  least,  the  direction  of  new  walls  is  profoundly 
influenced  by  the  concentration  of  the  surrounding  medium. 
In  general,  all  vital  processes  are  retarded  in  concentrated 
solutions.     Reproduction,  being  a  peculiar  form  of  cell  divis- 
ion, appears  in  some  cases  to  be  entirely  dependent  upon  the 
osmotic  pressure  of  the  surrounding  medium.     Irritability  is 
also  greatly  influenced  by  external  pressure.     Not  only  is 
this  function  retarded  in  concentrated  solutions,  but  in  some 
^  forms  the  direction  of  response  to  a  given  stimulus  may  be 
^  ?J  reversed  by  a  sudden  change  in  the  osmotic  surroundings. 
^  xj  The  comparative  concentration  of  the  external  and  internal 
^  solutions  acts,  in  many  cases,  as  a  stimulus  upon  the  organ- 
£  ism,  giving  rise  to  the  phenomena  of  osmotaxis. 

All  the  effects  of  high  concentration  of  the  surrounding 
liquid  seem  to  be  due  to  extraction  of  water  from  the  living 
cells.  They  may  be  due  either  to  a  drying-out  process  or 
to  decrease  in  turgidity.  That  they  are  sometimes  due  to 
the  former  is  proved  by  curious  analogies  between  the  vari- 
ous processes  which  extract  water  from  the  protoplasm. 
Whether  or  not  this  extraction  of  water  from  the  protoplasm 
itself  is  the  direct  cause  of  the  responses  to  concentrated 
solutions,  is  not  yet  known.  The  effect  may  be  a  chemical 
one,  due  to  the  increased  concentration  of  the  contained 
solutions. 


f 


INDEX 


Absorption  :  of  gases,  115 ;  of  solids  and 

liquids,  118;  of  solutes,  115. 
Acids  :  influence  of,  on  permeability,  61, 

74 ;  penetrating  power  of,  64. 
Acetanilid,  in  plasmolysis,  63. 
Acetone,  in  plasmolysis,  63. 
Action  of  protoplasmic  membrane,  80. 
Alcohol,  ethyl,  in  plasmolysis,  63. 
Alcohols:    penetrating   power  of,  71; 

aliphatic,  in  plasmolysis,  63. 
Alkalies,  penetrating  power  of,  64. 
Ammonia,  penetrating  power  of,  64. 
Ammonium  carbonate,  77. 
Ammonium  chlorid,  ionization  of,  23. 
Amphitrite,  eggs  of,  130. 
Amylase,  penetrating  power  of,  71. 
Anesthetics,  effect  of,  on  permeability, 

78. 
Anilin,  in  plasmolysis,  63. 
Anilin  dyes,  penetrating  power  of,  66. 
Animal  cells,  permeability  of,  64. 
Antipyrin,  penetrating  power  of,  64. 
Apple,  pressure  of  sap,  86. 
Arbacia,  eggs  of,  128. 
Arengo,  exudation  pressure  of,  102. 
Arrhenius,  18, 24. 
Artari,  70. 
askenasy,  111. 

Aspergillus:   permeability  of,  67;   so- 
lutes of,  83. 
Asterias,  eggs  of,  129, 130. 
Asci,  bursting  of,  54. 
Atmosphere,  internal,  120. 
Atomic  theory,  3. 
Avogadro,  principle  of,  11. 
Bacteria,  plasmolysis  of,  58,  61. 
Bacterium  termo,  plasmolysis  of,  61. 
Bases,  influence  of,  on  permeability,  61, 

74. 
Basidiobolus,  in 'osmotic  solutions,  132, 

138. 
Bean,  permeability  of,  65. 
Beauverie,  137. 
Beckmann,  37,  39. 
Beet:  permeability  of,  62,  64,  78;  solutes 

of,  83. 
Begonia,  permeability  of,  62,  73,  76. 
Berberis,  pulvini  of  stamens  in,  76. 
Blackman,  116, 117. 
Bleeding,  102 ;  theory  of,  104. 


Blood  corpuscles,  54,  57. 

Blue,  methyl,  penetrating  power  of,  66, 

77. 
BOHM,  111. 

Bonnier,  73,  98. 

Bouilhac,  70. 

Bourget,69,  119. 

Bower,  88. 

Boyle,  principle  of,  10. 

Browne  and  Escombe,  116. 

buffum  and  slosson,  113. 

burgarszky,  18. 

Bursting  of  cells,  54. 

Cabbage,  pressure  of  sap,  86. 

Caffein:  in  plasmolysis,  63;  penetrating 

power,  64,  77. 
Calcium  :   absorption  of,  .67,  120 ;  pene- 
trating power  of,  69. 
Calcium  nitrate,  absorption  of,  120. 
Campbell,  66. 

Caoutchouc,  membrane  of,  82. 
Carbon  dioxid:   absorption  of,  93,  115; 
from  roots,  72;  influence  of,  on  trans- 
piration, 113;  penetrating  power  of,  70. 
Carbonates,  penetrating  power  of,  64. 
Carrot,  pressure  of  sap,  86. 
Celery,  pressure  of  sap,  86. 
Cell  wall,  permeability  of,  55. 
Chaetomorpha,  permeability  of,  62. 
Chaetopterus, chemical  fertilization  of, 

130. 
Chemical  theory  of  semi-permeabil- 
ity, 82. 

Chenopodiaceae,    absorption  of  iodin 
by,  69. 

Chlorids,  penetrating  power  of,  78. 

Chromulina,  reversal  of  tropism  in,  140. 

Chloral  hydrate,  in  plasmolysis,  63. 

Cholesterin,  81. 

Cilia,  in  osmotic  solutions,  139. 

Clausen,  116. 

Cocos,  exudation  pressure  of,  102. 

Codium,  permeability  of,  78. 

Coefficients,  isosmotic,  56. 

Cohnheim,  83. 

Colloids,  27,  49. 

Conductivity  of- saps,  85. 

Copeland,  77,  83,  88,  105,  108,  111. 

COPELAND  AND  KAHLENBERG,  69. 

Copepods,  reversal  of  tropism  in,  139. 


145 


146 


Diffusion  and  Osmotic  Pressure 


Copper,  accumulation  of,  69. 

Copper  ferrocyanid  membrane,  82, 112. 

Copper  sulfate,  110. 

Coupin,  68. 

Crystalloids,  49. 

Curcuma,  permeability  of,  62. 

Curtis,  54. 

Curvature,  role  of  turgidity  in,  89. 

Cynara,  78. 

Czapek,  72,  74. 

Dandeno,  22,  68,  71,  72,  98. 

Davenport,  31, 128. 

Death,  theory  of,  75. 

Demoussy,  67, 119. 

Dero  vaga,  regeneration  of,  128. 

Devaux,  69. 

De  Vries,  55,  56,  61,  62,  64,  65,  74,  77,  83,  84. 

Diffusion,  of  gases,  9. 

Diffusion  tension,  of  solvent,  30. 

Digestion,  outside  the  body,  71,  72. 

Dixon,  111,  112. 

dutrochet,  111. 

Dyes,  anilin,  penetrating  power  of,  66. 

Ectoplast,  51,  53,  80,  82. 

Eggs,  parthenogenesis  of,  by  cold,  143. 

Electrolytes,  in  plasmolysis,  56. 

Elodea,  permeability  of,  66. 

Embryo,  permeability  of,  72. 

Endosperm,  permeability  of,  72. 

Engelmann,  139. 

Environmental  factors,  47. 

Enzymes,  penetrating  power  of,  71,  72. 

Epidermis,  permeability  of,  117. 

ESCHENHAGEN,  131. 

Ether,  ethyl,  in  plasmolysis,  63. 

Eerera,  59. 

Euphorbia,  nectaries  of,  79. 

Evaporation,  110. 

Exudation  :  nature  of,  73;  from  wounds, 
102;  from  glands,  71,96. 

Exudation  pressure,  theory  of,  104. 

Fagopyrum,  solutes  of,  83. 

Fehling's  solution,  65. 

Ferric  pyrolignate,  110. 

Fertilization  by  cold,  143. 

Fick,  18. 

Filter  theory  of  semipermeability, 
80. 

Flusin,  82. 

Form,  retention  of,  87. 

Formaldehyde,  63. 

Frazeur,  128. 

Fuchs,  104. 

Fucus,  permeability  of,  74. 

Fungi,  in  osmotic  solutions,  138. 


Furfurol,  63. 

Gases:  diffusion  tension  of,  9;  mixed, 
11 ;  absorption  of,  115 ;  transmission  of, 
120. 

Gay-Lussac,  principle  of,  10. 

Glycerin,  in  plasmolysis,  56,  62,  63,  64, 

67,  77,  79. 

Glucose:    in    turgor,   84;    penetrating 

power  of,  61,  65,  67,  70,  79. 
Godlewski,  108. 
gogorza  and  gonzalez,  54. 
Graham,  18. 
Gram  molecule,  20. 
Greeley,  75, 143. 
Growth,  role  of  turgidity  in,  88. 
Gryns,  57. 

Gunnera,  solutes  of,  83. 
Guttation,  73,  74,  98. 
Hamburger,  57,  83. 
Hansteen,  66. 
Hartig,  110. 
Haupt,  79, 100. 

Hearts  :  in  osmotic  solutions,  129 ;  effect 
of  cold  upon,  143. 

Heald,  69,  85. 

Hedin,  57,  83. 

Helianthus:    permeability   of,   65,   74; 
solutes  of,  83,  84. 

Hilburg,  78. 

HOber,  18,  83. 

Honey-dew,  98. 

Imbibition,  93. 

Infusoria,  in  osmotic  solutions,  132. 

Internal  atmosphere,  120. 

Iodin,  penetrating  power  of,  68,  69. 

Ions,  in  absorption,  53. 

Ionization:  of  gases,  23;  of  solutes  in 
liquids,  24. 

Irritability,  changes  of,  139. 

Jarius,  130. 

Janse,  62,  65,  77, 108. 

Jennings,  63. 

Jumelle,  112. 

Jung,  128. 

Kahlenberg,  22. 

Kinetic  theory  of  matter,  4. 

Klebs,  62,  88. 

Knop's  nutrient  solution,  132. 

kohlrausch,  42. 

KOHLRAUSCH  AND  HOLBORN,  42,  43. 

Koppe,  57. 

KOSSAROFF,  113. 

KOvesi,  83. 
Krabbe,  75. 
Kraus,  C,  102. 
Kraus,  G.,  83. 


Index 


147 


Laurent,  67,  71. 

Leaves  :  absorption  by,  68,  103 ;  exuda- 
tion from,  99;  guttation  of,  74;  permea- 
bility of,  71. 

Lecethins,  81. 

Lehman,  69. 

Leitzmann,  116. 

Lemna,  permeability  of,  76. 

Lice,  reversal  of  tropism  in,  141. 

Lidforss,  54. 

Liebermann,  18. 

LlLIACE^E,  69. 

Liquids  :  absorption  of,  118 :  diffusion 
of,  12, 13. 

Livingston,  132, 138, 143. 

LOb,  57. 

Loeb,  53,  74,  128, 129, 139. 

Loeb,  Fischer,  and  Neilson,  130. 

Lupinus:  in  osmotic  solutions,  130;  so- 
lutes of,  84. 

MacDougal,  69, 119. 

Maquenne,  74,  84,  85. 

Massart,  61, 140, 141. 

Matruchot  and  Molliard,  70,  75. 

Matter:  nature  of,  3;  states  of,  6. 

Mayer,  84. 

Medium,  influence  of,  124. 

Meerburg,  36,  82. 

Membranes  :  cellulose,  51 ;  copper  f erro- 
cyanid,  82. 

Membranes:  protoplasmic,  49;  action 
of,  80. 

Mercuric  chlorid  :  effect  on  permea- 
bility, 74 ;  penetrating  power,  68. 

Methyl  blue,  penetrating  power  of,  66, 

77. 
Methyl  cyanid,  in  plasmolysis,  63. 
Methyl  vtolet,  penetratingpower  of,  66. 
Mimosa,  pulvini  of,  77. 
Mohl,  98. 

Molds,  bursting  of,  54. 
Molisch,  65,  72,  75, 102, 103, 106. 
Morgan,  129. 
Morse  and  Horn,  36. 
Mccor,  exudation  from,  99. 
Muller,  116. 

Muscle,  permeability  of,  74. 
Myriotonie,  59. 
Naccari,  18. 
Nathansohn,  78. 

Nectaries:  artificial, 99;  conditions  for 
secretion  of,  101 ;  theory  of,  96. 

Nernst,  18. 

Nernst-Palmer,  37,  38,  40. 

Nitrates,  test  for,  65. 

Noll,  54. 


Nostoc,  permeability  of,  70. 

Nuclei:  frozen,  dried,  etc.,  76;  in  osmo- 
tic solutions,  128. 
Oltmanns,  74,  102. 

Onion  :  permeability  of,  78 ;  solutes  of,  83. 
Ono,  69, 125. 
Osmotaxis,  140. 

Osmotic  pressure:  in  general,  28;  de- 
monstration of,  32;  of  electrolytes,  27; 
of  non-electrolytes,  25;  indirect  meas- 
urement, by  freezing-point,  37;  by  boil- 
ing-point, 38;  by  vapor  tension,  39; 
calculation  of,  for  electrolytes,  42;  for 
non-electrolytes,  41;  compared  to  dry- 
ing, 141. 

Ostwald,  42. 

Ostwald-Walker,  16,  40,  42. 

Overton,  63,  71,  81. 

Oxygen:  absorption  of,  115;  effect  of, 
on  permeability,  78;  penetrating  power 
of,  70. 

PARAMOsciA,?permeability  of,  63. 

Parthenogenesis:  129;  by  cold,  143. 

Pear,  pressure  of  sap  of,  86. 

Peas,  solutes  of,  84. 

Penicillium,  in  osmotic  solutions,  131. 

Permeability  of  protoplasm:  60,  64, 
67,  68,  69,  70, 118;  outward,  63,  71 ;  varia- 
tions in,  72 ;  effect  of  on  turgidity,  86. 

Pfeffer,  35,  50,  55, 64,  66, 68,  77,  78, 104, 105, 
120. 

Phaseolus:  in  osmotic  solutions,  130, 
138 ;  permeability  of,  65,  78 ;  solutes  of, 
83. 

Phenol,  in  plasmolysis,  63. 

Phloroglucin,  in  plasmolysis,  63. 

Phosphoric  acid,  from  roots,  72. 

Photosynthesis,  117. 

Phytolocca,  penetrating  power  of  sap 
of,  98. 

Picric  acid,  110. 

Pilobolcs,  bursting  of,  54. 

Pisum:  in  osmotic  solutions,  130,  138; 
solutes  of,  83,  84. 

Pitra,  103. 

Plant  lice,  reversal  of  tropism  in,  142. 

Plasmolysis:  in  general,  54,  60;  by  cold, 

75;  of  bacteria,  58;  effect  of  on  growth, 

88;  on  permeability,  74. 

Platinum  chlorid,  65. 

Poisons,  permeating  power  of,  68. 

Pollen  grains,  bursting  of,  54. 

Polygordius,  reversal  of  tropism  in, 
139. 

Polimorphism  :  in  Basidiobolus,  137;  in 
fungi,  138;  in  Stigeoclonium,  132. 

Potassium:  absorption  of,  67, 120;  pene- 
trating power  of,  69. 

Potassium  chlorid,  in  turgor,  83. 


148 


Diffusion  and  Osmotic  Pressure 


Potassium  nitrate  :  absorption  of,  120 ; 
in  plasmolysis,  56,  61;  in  turgor,  83; 
penetrating  power  of,  65,  68,  74,  77. 

Pressure:  exudation,  102;  gas,  9;  os- 
motic (see  Osmotic  pressure). 

Protoplasm,  49. 

Protoplasmic  membranes,  action  of,  80. 

Puriewitch,  78. 

Quercus,  copper  in,  69. 

Quincke,  82. 

Raciboeski,  132, 137, 138. 

Reinhardt,  88. 

Reproduction,  in  osmotic  solutions,  138. 

Richards,  68. 

Richtee,  139. 

Rise  of  water,  107. 

Root  hairs,  exudation  from,  75. 

Roots,  permeability  of,  69. 

Rothert,  140. 

Salts,  inorganic,  penetrating  power  of, 

67,  71. 
Sambucus,  permeability  of,  76. 

Sap  :  cell,  52 ;  expressed,  conductivity  of, 

85. 

Sargent,  128. 

Scheffer,  18. 

Schneider,  112. 

schwendener,  76. 

Seeds,  in  osmotic  solutions,  130, 131. 

Selective  power,  118. 

Siphoned,  bursting  of,  54. 

Skertschley,  69. 

Slosson,  131. 

Sodium,  penetrating  power  of,  69. 

Sodium  chlorid  :  in  plasmolysis,  61 ; 
penetrating  power  of,  68,  74;  in  me- 
dium, 137. 

Sodium  nitrate:  absorption  of,  120; 
penetrating  power  of,  79. 

Solids,  absorption  of,  118 ;  diffusion  of, 
14. 

Solanum,  absorption  by,  69. 

Solutes:  absorption  of,  115:  active,  na- 
ture of,  83 ;  transmission  or,  120. 

Solution,  Knop's,  132. 

Solutions:  denned,  16;  properties  of, 
124;  molecular,  21 ;  normal,  21;  of  gases 
in  liquids,  17;  of  liquids  in  liquids,  16; 
of  solids  in  liquids,  18;  terminology 
for,  20. 

Solution  theory  of  semipermeabil- 
ity,  80. 

Sperms,  in  osmotic  solutions,  129, 139. 
Spirogyra  :  cold  plasmolysis  of,  75,  141 ; 

permeability  of,  62,  65.  68. 
Stahl,  140. 
Stange,  84, 130. 
Stentor,  cold  plasmolysis  of,  75, 143. 


Stevens,  69. 

Stichococcus,  permeability  of,  70. 

Stigeoclonium  :   drying  out  of,  143;  in 

osmotic  solutions,  132,  138;  zoospores 

in  solutions,  139. 

Stomata,  93. 

Strasburger,  110. 

Stratiotes,  permeability  of,  62. 

Strongylocentutus,  eggs  of,  129. 

Sucrase,  penetrating  power  of,  71. 

Sugar,  cane:  in  plasmolysis,  61;  pene- 
trating power  of,  65. 

Sunflower,  permeabilty  of,  65. 

Support,  mechanical,  by  turgidity,  87. 

Surrounding  medium,  influence  of,  124. 

Sutherst,  85. 

Tamman,  36. 

Temperature:  influence  of,  on  absorp- 
tion, 95;  influence  of,  on  permeabilty, 
75 ;  influence  of,  on  turgor,  77. 

Tension,  diffusion,  of  solvent,  30. 

Theories:  of  matter,  3;  of  semipermea- 

bility,  82. 
Thunbergia,  permeability  of,  71. 
Thuya,  transpiration  of,  111. 
Tonie,  59. 
Tonoplast,  52,  53,  80,  82. 

Tradescantia,  permeability  of,  56,  62, 
76. 

Transmission:  of  solutes,  115,  120;  of 
water,  95. 

Transpiration,  93,  96. 

Transpiration  stream,  107. 

Traube,  50. 

Tropism,  reversal  of,  139. 

True,  68, 137. 

turgescence,  52. 

Turgidity:  nature  of,  49,  52,  53;  main- 
tenance of,  86, 121 ;  relation  of,  to  activ- 
ity, 87. 

Turgor,  52,  55,  74. 

Turnip,  pressure  of  sap  of,  85. 

Urea:  in  plasmolysis,  62;  penetrating 
power  of,  77,  79. 

Vacuole,  52. 

Vandeevelde,  131. 

Van  Rysselberghe,  76, 134. 

Van't  Hoff,  25,  37,  40. 

Vegetable  marrow,  pressure  of  sap  of, 
85. 

Vesque,  95. 

Vicia  :  nectaries  of,  79 ;  roots  of,  65. 

Violet,  methyl,  penetrating  power  of, 
66. 

voigtlander,  18. 

Von  Mayenburg,  67,  83, 118. 

Walden,  36. 


Index 


149 


Walker,  40. 

Wall,  cell,  51.  52. 

Water  :  absorption  of,  by  plant  cells,  91 ; 
absorption  of,  by  muscle,  53 :  in  cellu- 
lose wall,  52 ;  loss  of,  95 ;  transmission 
of,  95. 

Water  pores,  74,  96. 

Westermeier,  108. 

Whetham,  43. 

Wielee,  65, 102, 108. 


wlesner  and  molisch,  116. 

Wilson,  98. 

Wladimiroff,  58. 

Woods,  112. 

wortmann,  66. 

Yasuda,  132. 

Zea,  solutes  of,  83. 

Zoospores,  in  osmotic  solutions,  139. 


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